Team/Multi-User Best Practices

Think like this,
1 user belongs to a "Team of 1"
5 user belong to a "Team of 5".
So your app's main entity (or main customer) is basically; "a Team". People pay for having a "team" in your app.

Very simple database structure is (assuming you are using a relational DB):

Team > id, max_member (assume you have different plans for each team size), created_at, payment_info ... etc

User > id, team_id, name, email, etc....

Now you can subscribe each of your user to a different team. (They are first member of that team)
They can invite (let's say via email) other people
if "number_of_current_members_of_the_team < max_member"

You can create roles for team (just put another column to user table, team_role).
Now you can show a link in the UI if user_role == 1 (let's say it stands for admin role) and by following this link they can reach to team settings page so they can add/remove new users, update billing info etc..

I hope this answers your question, and helps