There's nothing wrong with latching on to a passing group. The more, the merrier.

However, keep in mind that if you do join the group, be prepared to pull when it's your turn. Also, make sure you know the etiquette rules of group riding (hold your line, call out obstacles/debris etc) because chances are, the group you've just joined has been riding together for a while and are accustomed to each other's style etc. There's nothing (or at least to me) more nerve-wracking then having inexperienced riders join a group who have no knowledge of what to do in pacelines/group rides and don't ask and don't point stuff out or hold their line.

Everyone has to start somewhere so don't be afraid to ask if you're not sure how to signal/when to signal, how to pull etc etc.

Enjoy the group rides!