At my school I used to be in athletics class even though I didn’t sign up or paid to be there, I just simply started acting as if I was part of the team. How would you call a person like that?
An interloper, perhaps?
interloper noun [ C ] disapproving
someone who becomes involved in an activity or a social group without being asked, or enters a place without permission:
- Security did not prevent an interloper from getting onto the stage at the opening ceremony.
The related activity of taking advantage of what is free is sometimes called freeloading and the person who does it is called a freeloader.
From the Oxford English Dictionary:
colloq. (orig. U.S.).
Originally: a person who takes full advantage of the free food or drink available at a public occasion. Later more generally: a person who partakes of something offered free of charge without giving anything in return; a sponger, a scrounger.
That said, I would use the word a little more broadly to potentially include your use case as well, as one of the OED's own examples shows it being used:
1979 D. Adams Hitch Hiker's Guide to Galaxy vi. 48 I didn't become captain of a Vogon constructor ship simply so I could turn it into a taxi service for a load of degenerate freeloaders.
The context of this quote makes it clear that they are not simply "taking advantage of free food or drink available", but that they have trespassed by sneaking surreptitiously on board as stowaways (to allude to some of the other answers' more specific terms for this activity in the case of a ship).
First of all I see from our instruments that we have a couple of hitchhikers aboard. Hello wherever you are. I just want to make it totally clear that you are not at all welcome. I worked hard to get where I am today, and I didn’t become captain of a Vogon constructor ship simply so I could turn it into a taxi service for a load of degenerate freeloaders. I have sent out a search party, and as soon as they find you I will put you off the ship. If you’re very lucky I might read you some of my poetry first.
— Hitchhiker's Guide to the Galaxy, Chapter 6
Edit: I also like (perhaps prefer) Acccumulation's "free rider", which is at least as appropriate in this context in that it suggests illegitimate entry.
There are a number of possible words that apply, some more picturesque than others.
To "impersonate" means to pretend to be someone you are not:
She impersonated a student in the class in order to get free instruction.
"Pose" also works:
She posed as a club member in order to eat at their free buffet.
"Fake" (or "pretend") is a common term for this:
To the casual observer, she was just another student on campus, but she was only faking/pretending in order to take classes without paying tuition.
"Sneak in" can be used to indicate illicit and furtive activity:
She would often pretend to be an usher in order to sneak into the concert hall and watch performances for free.
Other possibilities: infiltrate, disguise (oneself) as, insinuate into, worm into, put up a front, playact, feign, etc.
This only works in an academic/classroom context, but you could say you were "auditing" the class (in contrast to those on the class roster).
Of course, most schools actually have policies about auditing (permission of the instructor, etc.), so if you haven't actually obtained the proper permission, then "auditing" is just a euphemism for "sneaking into class".
EDIT: To expand on my "euphemism" comment above, clearly sneaking in without permission is not actual (legitimate) auditing, per the rules of whatever school or class we're talking about. If I were writing a descriptive essay, I probably wouldn't use this term, unless I was quoting the excuse the student gave when he or she was caught.
But the term might still be well understood as "attending class for no credit", especially with other cues. Here's an example:
I run into a friend who says they saw me with a group at the local university's tennis courts. They ask, "I thought you already graduated, are you back in school?"
I might respond, "Nah, I'm just (fingerquotes) auditing the class".
The "fingerquotes" (or just perhaps a certain vocal intonation) are a hint that I'm using the term euphemistically, and the most plausible interpretation of that in this context is that I'm unofficially (without approval) attending the class; i.e. just sneaking in.
How about trespasser?
While the usual sense is in entering into private land, it also applies to any restricted spaces, such as a classroom or country club.
1.b - a wrongful entry upon the lands of another
2 - an encroachment or intrusion
- to encroach on a person's privacy, time, etc.; infringe (usually followed by on or upon).
Andrew provided a good selection of verbs that could apply to the act being described, as well as "masquerade" suggested by Tyler, but since the question asks "How would you call a person like that?" I'm going to try suggesting some adjectives and nouns.
First up, Fake as an adjective would be useful. For example if a person named Janet attended a class that was supposed to be paid, but she didn't pay for it, then you could say
Janet is a fake Student
This also applies to other situations, such as being a Fake member of a country club. On the other hand while Fake could be applied directly to Janet (Janet is fake) or used as a noun (Janet is a fake), I would interpret those statements as referring to more general character concepts than specifically sneaking into exclusive events.
In terms of nouns, one that would often work would be Impostor. There is some connotation of pretending to be a specific person (It wasn't Janet at all, but an impostor!) but it's just as valid to apply to someone pretending to more generally be part of a certain group or the like (He wasn't a student at all, but an impostor!).
Describing the action rather than the person does seem to be easier though, and allows for more nuance in regards to how they were attempting to fit in and why they were doing so. Someone who attends an event uninvited with the intent to subtly cause trouble might infiltrate, while one who also wants to cause more overt trouble at an uninvited event might crash it. But, when asked to describe "a person who deceitfully participated in an event" as "fake" or an "impostor" if there is no other detail.
For the specific case of sneaking into additional movies, we used to call it theater hopping--where you pay for one film and watch as many as you can stomach.
In a way that's the same thing you are doing, paying for some classes at the school but then attending a few extra classes for free--but I doubt "class hopping" will ever catch on.
No one has yet suggested surreptitiously?
Which means to do something in a way that attempts to avoid notice or attention; secretively.
The student surreptitiously joined the team
You can also be called an:
verb (used with object), in·trud·ed, in·trud·ing.
to thrust or bring in without invitation, permission, or welcome.
Geology.to thrust or force into.
You could say loitering? Here's the definition;
"stand or wait around without apparent purpose"
Another fitting word, which is also better, would be masquerading.
"be disguised or passed off as something else"
"You 'masqueraded' as a member of the athletic class."
This sentence works, because of whatever situation put you there, you still played along as one of the classmates.