Just loop through all players and add each one to the menu like so
PHP Code:
decl String:sName[MAX_NAME_LENGTH], String:sUserId[10];
for(new i=1;i<=MaxClients;i++)
{
if(IsClientInGame(i))
{
GetClientName(i, sName, sizeof(sName));
IntToString(GetClientUserId(i), sUserId, sizeof(sUserId));
AddMenuItem(menu, sUserId, sName);
}
}