Make sure everyone has as much magic resist as they can pack on. Dragon roar will be the doom of the group if people are feared they can't heal/disc/etc.
Tank, Cleric preferably, Enchanter for clarity+group MR+haste, rogue/monk dps a couple of them. Shaman is optional mostly acts as a spot healer along with any other class that can spot heal.
Should be good to go. Like Valdain said 1.5-2 groups.
http://wiki.project1999.com/Dragon_Roar
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I've seen it done with 5 velious twinked players in person.