Is it right to say "you wanna join?" to invite someone to join me for the activity? Is it causal English or not grammatical at all. thanks. I know it's correct to say "you wanna join me?", however i wonder if "you wanna join" works as well here. Thank you
The word join can mean DO TOGETHER or BECOME A MEMBER. If you want to use the DO TOGETHER meaning, it requires an object, for example join me or join us, or a preposition join with....
The BECOME A MEMBER meaning can be used without an object, as long as it's clear what group or organization somebody became a member of:
It's a great club. Why don't you join?
I would therefore assume that "You wanna join?" was asking whether I wanted to become a member of some club or organization.
Another option to consider is join in. You would use that if you are part of a group of people doing something, and wish to invite another person to participate.