This is a low-pressure way to invite someone to do something with you:
A: I’m going furniture shopping on the East Side this weekend if you want to join me.
B: Sure, I’d love to!
To “join” someone means to do an activity together. We use “if you want to join me” as an indirect offer. The real meaning is “...and if you want to join me, you are welcome to.”
Instead of "join me", you can say “come” or “come along”:
I’m going to their concert next week if you want to come.