Navigate to Monthly Billing | Access Management > User Groups.
Select the MSSP from the drop-down menu.
Select the required User Group from the menu.
Use Search option to filter the required user group from the list if required. If you want to create a new user group, follow Creating a New User Group.
In the Users section, click + Add User.
Check the Available users boxes from the list and click Add for existing users.
Click Invite New User and follow Inviting a New User to invite and add a new user if users are not available in the list.