"Exactly," said Harry. "And we know you get in using those funny coins, or tokens, or whatever they are, because I saw that witch borrowing one from here friend -"
Harry Potter and the Deathly Hallows
I looked up "get in", but it seems to me that none of definitions could fit for the context. If I have to guess, I think it probably means to succeed in doing something. However, I don't know if I'm getting it right. Is it a common usage? What does it truly mean?
In the sentence in the question, get in is not a set phrase (or phrasal verb). It is simply two words put next to each other.
If looking it up in a dictionary, you would look up the verb get and the preposition in.
In this case, in means the same thing as inside, so the sentence could be paraphrased:
"And we know you get inside [the room/place] using those funny coins …"
The above sense of in should not be confused with the more colloquial the in crowd, you should get into the movie, or I'm in on the bet, none of which uses in as a preposition of physical location, but instead as a figurative expression of joining or being aware of something.
Unfortunately, knowing the difference in cases like this is often a matter of familiarity, use, and context, rather than something that can be quickly determined by looking it up in a dictionary.
To "get in" means to successfully enter. Then "... using those funny coins" is a participle phrase describing how to get in
You can get in through the back door, which is unlocked.
You get in most houses using a key.
