There are four types of member roles on Motion - the listed Owner, Admins, Collaborators, and Guests.
Owners, Admins, and Collaborators can create and edit reports, while Guests can view reports.
Owners and Admins can add new workspaces and ad accounts, as well as add new members to Motion.
You can add unlimited Guests on any Motion plan. They won't count towards your total member count.
To add new users:
Head to the bottom left menu
Select 'My account'
Choose the 'Add New Members' button in the top right corner
Add the email address for the new user account, and select the role, then click 'Send Invite'