[CyberSnaak]

pawn Код:

dcmd_joindm(playerid, params[])
{
    if(IsPlayerInDM[playerid] == true)
    {
        SendClientMessage(playerid,-1,"You are already in a DM event!");
    }
    else
    {
        SetPlayerPos(playerid, POS HERE);
                SetPlayerInterior(playerid, InteriorIDHERE);
        SendClientMessage(playerid,-1,"You are not in a DM event, Good luck!");
        IsPlayerInDM[playerid] = false;
    }
    return 1;
}

Something like this?

P.S: just reverse it slightly, Instead of IsPlayerInDM[playerid] true , change true to False on the /leavedm command.