16 people on each side. There's no need to have a team. The people involved are those who registered for the raid.
A team is handy to arrange though when attacking, as team members will always respawn at the same point in a raid (attackers respawn at any corridor between rooms).
The team spawning at the same place is currently bugged I think but fixed in a patch to some.... "soon" 
Well DB is up for some insta raid action if anyone wants
This a quick post...How many teams from the same SG take part on a base raid?
Just wondering as we'll soon be doing one and already have a full team and other people are intrested.