Generically, banda (BANN-Duh, lit. band) can refer to a group of people or a crowd. One's banda is the group of people that that person holds dearest. Bandera (Bann-THE-rah, lit. flag) is equivalent to banda but significantly more colorful.
Había un huevo de banda.
[It was a darn big crowd]
Traete a tu banda a los quinceaños de mi prima.
[Bring your friends and family to my cousin's quinceanera party]