new // global
    bool: MissionStarted,
    bool: IsPlayerOnMission[MAX_PLAYERS];

// In your command or in your join code
    if(!MissionStarted)
        MissionStarted = true;
        SetTimerEx("AbortMission", 25 * 1000, false, "i", playerid);
    }
    IsPlayerOnMission[playerid] = true;

//somewhere outside
forward AbortMission(starter);
public AbortMission(starter) {
    for(new i, count = -1; i != MAX_PLAYERS; ++i) {
        if(IsPlayerOnMission[i]) {
            if(++count) {
                // mission started
                return ;
            }
        }
    }
    GameTextForPlayer(starter, "Mission aborted", 4000, 3);
    IsPlayerOnMission[starter] = false;
    MissionStarted = false;
}