You can add users to the default groups (Administrator or Power User) only if you purchased the Professional edition. You can add your users to a group when you send them an invitation. However, if you forget to select the group during the invitation process, you can add your user to the relevant group later on.
Trados Studio cloud capabilities provisions 2 default groups in the Root folder (location): Administrators and Power Users. As administrator, you can invite users in these groups only if you purchased the Professional edition. You cannot create other groups.