Since there are players skilled enough to play 2-3 clients at same time and most of them have high level characters with good build, it would be nice if there's a feature where you can lock the number of members in a group you formed. I know it sounds selfish but my proposal also has consideration for those who play solo, specially those who are not on Premium Status. For example, there are people who farm Cave of Ulverick where they just kill the anansis, then just leave and the last player who joins the group would be left to wait for other players hoping for a cleric to join. Another example are those players just doing only daily dungeon quests in Sikuku Catacombs to farm for rune empowerment materials and players who join the group hoping for boss kills would be disappointed.
So, with a group locking feature, those who would now queue solo in PVM Dungeons would have higher chance of forming a group with other solo players or a group who is open or willing to allow other players to join and not wasting Game Arena Energy. It would also give players who enjoy playing with 3 clients 'solo' time without worrying of a possible player joining his group and ruining his gameplay... like those who often use stun in COU. This would also give you privacy when teaching a clanmate about different dungeons, specially the way around Sikuku Catacombs.