I just created a new Backend user account and the following message was pop-up:
Backend users can hold rights on multiple Backend groups.
When an administrator creates a Backend user account that already exists, the Backend will send an email invitation to the user:
Accepting an invitation
Users can accept any invitations to obtain new user rights. These invitation are shown by clicking on the figure icon in the upper right corner on the Backend:
New user right will be instantaneously granted when accepting the invitation: