There are two ways to enter regular players into a group. An admin can enter players as members of the group, when creating or editing the group; otherwise, when you enter the results of a match played with the new player, enter their email address in the appropriate field of their faction.
The new player will receive an invite to register on rankade, if they don't have an account yet, at that address.
Moreover, you can also create ghost users (i.e. users that are not linked to an email address) when creating or editing the group through the webapp.