How many seats does my plan include?
Unlimited on the Team plan, one on Supporter. A colleague costs nothing extra to add, so what your plan counts is profiles rather than people.
The Team plan does not charge per person, so the bill is the same whether the team is three people or thirty. Invite whatever mix of Members and Admins the work needs. Sending invitations is an admin job: the Team page appears in the sidebar when you are a team admin. Seats only exist on the Team plan, and a Supporter plan is one sign-in, so adding a colleague means moving to Team.
What your plan counts instead
Profiles. The Team plan starts at 20 profiles shared across the whole team, and profile packs add more whenever you need them. Why am I out of profile slots? covers what counts toward that number and how to get more room.
If an invitation will not send
Invitations are held back by profiles, not by seats. If the person you are inviting already has an Advocate360 account with profiles of their own, those profiles come with them into the team and count toward its total. When there is not enough room the invitation is not sent, and the message names how many profiles they are bringing and where the team stands. Add a profile pack, then send the invitation again. Inviting somebody who brings no profiles works even while the team is at its limit.
A refused invitation is not saved for later, so once there is room you send it again yourself. How do I add someone to my team? has the steps.