This guide will assume you have reached level 40 and have completed your second class change quests.
Every good group will consist of a mix of the following class types:
A perfect group will have a Shillen Elder and a Bishop. A group healer knows when to heal, not overheal to burn out his mana or cause the mob to hate him and most importantly, to keep everyone alive.
A perfect group will have a Prophet, who has various buffs for each classes (although doesn't have Empower, the very important skill for nukers.) A good prophet knows which skill to buff on whom and save his precious mana especially when being in a big party. If no Prophet, you may use a Warcryer or Overlord, who can do group buff by casting once (Warcryer for party, Overlord for clan and alliance.)
A perfect group will have a Swordsinger and a Bladedancer. These classes can be replaced with normal damage dealers but with a loss to group efficiency. Good singers and dancers must know which song/dance to sing and select the most proper ones due to their limited of mana, and they should keep the song/dance on all the time.
A perfect group will have a Spellsinger. A nuker is not essential and played wrong can be a drain to the group rather than a help. Played right a nuker is the groups crowd control. Use your biggest damage spell when mana permits otherwise just use mana for useful buffs/debuffs and crowd control. Nukers can help the party gain the extra experience by using the overhit skill to finish the mob.
A perfect group will have a mix of damage dealers. This mixing of damage dealers is the only way, all damage dealers of one type can impeded your damage output if you come up against [damage] resistant mobs. The mix isn't important , the important thing is to not stack your party one way or the other. If you do melee damage and haven't been mentioned here[by class], then you fall into this catergory.
A perfect group will have a Dark Avenger. Other tanks are the Shillen Knight and Templar Knight. Other classes may tank but again this will greatly decrease your effiecency as the lack of hate skills or shield begin to tell on the healer's MP.
A want-to-be-rich group will have a Bounty Hunter. Bounty Hunter can spoil extra items/materials from monsters with higher possibility than common dropping. The higher your level and monster's is, the richer and extravagant the items/materials you can gain. This proves to be extra adena that each player could gain by selling these items/materials.
Choose mobs that the group can handle a number of simultaneously. Choose a place with good respawn. As a group you should never have to rest, If you find the group running out of mana there are either more mobs than this group can handle or they are too tough for the group, either way you should change location.
There are always players that do not know how to play their class within a group environment it is up to you to point out errors they make, if they continue to cause the group mana loss do to bad play style/tactics then replace them or ask them to leave the group they are often more of a hindrance than a help if they cause mana loss.