There are two ways to get an act onto your roster.
Invite them. They get an email, create their own account, and fill in their own profile: photos, bio, travel radius, availability. This is the better route: the act keeps their own details up to date and you stop being the one maintaining them.
Add them yourself, if they aren't on GigSpace and you want to book them now. You can invite them later and they take ownership of the profile.
An act can be on several agencies' rosters at once. Their profile is shared; your bookings with them are yours alone and no other agency sees them.
You can archive an act from your roster at any time; see Archiving an act.