Every Football Association that is playing in the Silver 3 Division or above can participate in the One Team competition. 
Managers of an FA can participate in the exclusive competition by loaning their players to the One Team squad.
A loaned player stays as a crucial part of the One Team squad until the last One Team competition of the season is played, unless he is swapped in the meantime.

Even though the captain and the vice-captains pick the team and set the formation, player positioning, mentality, and tactics, all Managers contribute to the preparation level of the One Team squad