[TheDS1337]

then the check is not needed?

public OnClientPutInServer(int client)
{
    new String:SteamID[50];
    GetClientAuthString(client, SteamID, 50);
    if (StrEqual(SteamID, "STEAM_ID:H:ERE", false))
    {
        ServerCommand("sm_invite #%i 10", GetClientUserId(client)); 
    }
}

-->

public OnClientPutInServer(int client)
{
    ServerCommand("sm_invite #%i 10", GetClientUserId(client));    
}