You can add or remove users within your account:
- Switch to the specific team account.
- Select "Teams" in the menu.
- Scroll down a little bit, now the team members will be visible.
- Invite new members by adding their email address.
- Remove existing members by clicking on the remove button.
Repeat the above if you have multiple team accounts.
To upgrade your number of licences have a look here: Upgrade your licences.