If someone already has a login at your club, either as a player or someone on the staff list, you can appoint them as a team admin directly from the players or staff list. It's a multi-select of teams rather than a single dropdown, so someone running both the 1st XI and the Vets can be appointed to both at once, in one action.
If the person you want to bring in doesn't have a Dugout login yet, appointing them directly isn't the right path. Send them an invite instead (see Joining a club you've been invited to), and appoint them once they've accepted.
Team admins you appoint can manage fixtures, squads and stats for the teams they're scoped to, but never billing or club-wide settings. Those stay with Club Admins and the Owner.