ALL RUGBY UNION PLAYERS, ADULT or U18s MUST REGISTER THEMSELVES ON THE RFU GAME MANAGEMENT SYSTEM (GMS) ON AN ANNUAL BASIS

To register with the RFU, follow these steps:
Annual Registration all players in all formats of the game will be required to register themselves annually to play rugby within RFU member clubs. LINK

Online Process:
Players will complete a simple online registration process each season using their GMS account.
After registration, a Club Registrar will approve it via GMS. LINK2

Type of Registration:
The type of RFU registration needed depends on the competition and leagues that the club competes in. Players may need to attach or sign documents to support their registration. LINK3

For more detailed information, you can visit the official RFU website.