Invites are created by an administrator: in Settings open “Invites”, tap the “+”, choose the “Assigned role” and tap “Create invite”. Then, with “Share”, send the code to the team chat: whoever receives it enters it in the app and joins.
Team-mates who don’t use the app can still be added to the roster, from Settings → “Players” → “+”. When they arrive, they will link their account to that card.