Within TriFact365, there are 2 options for assigning a User to an administration.
Go to Settings (Cogwheel) > Administrations > Pencil > Users. Then select one or more Users under Available Users and move them to Assigned Users by clicking on the Arrow (>).
Go to Settings (Cogwheel) > Users > Pencil. Then select one or more Administrations under Available Administrations and move them to Assigned Administrations by clicking the Arrow (>).