Every person who touches the POS or the Merchant Portal needs a user account, and what they can do is controlled by the group they are in.
Staff need a user account to access the POS and the Merchant Portal. Add and edit accounts from the Users page in the Merchant Portal.
The Genius Back Office Portal controls which users have access to the Point of Sale app and to the Back Office Portal. Groups are where that is decided.
Source: Global Payments Genius Help Center for Genius for Retail. Confirm menu paths against your own portal, as some options vary by package and region.