Managing team members requires a Plus subscription and a role with sufficient permissions. See all roles.
Invite a user
To invite a user, click the main menu and select Manage Members.
Select the Invitations tab.
Click the Invite Member button at the top right.
User number limitations
The number of users you can invite depends on the subscription type. The Plus plan allows a limit of 5 users. To increase the number you will need an upgrade or an optional user package.