The good and bad can sometimes depend a lot on the character and morals of those to whom you are trying to belong.
Bad things:
- Peer pressure to do negative and antisocial activities - drugs, smoking, drinking, graffitti, crime etc.
- Need to wear the same clothing as others - which could be expensive and impractical and against the rules (especially in some contexts, eg school)
- Shutting out others who may be outside the "in" crowd but may be better and more loyal friends in the end (Actually, the movie "Sky High" just came to mind)
- Pursuit of activities in order the gather money in order to achieve belonging in certain crowds eg over working, crime
- Being used by others
Good things:
- Positive peer pressure to give up negative and antisocial activities
- Self worth
- True friendship
- Being able to get out and do stuff
That's just off the top of my head.