Prior to 50 the Reaver, like most hybrids, doesn't have a real role in a group. You can do decent damage, but not as much as an Armsman or a Mercenary (even though your melee damage is augmented by Lifetaps). Your buffs are self-only. Your chants are only worth using in specific situations. You’re just another body, and could be replaced by just about anything. While it sounds bad, the Reaver isn't in that much trouble. Although you won't be sought out for a group most of the time, neither will you be summarily rejected very often. Additionally, the Reaver does have a niche - in melee-heavy groups, in powerleveling, and against Epic level monsters.

After hitting 50, a Flexible Reaver with Leviathan is one of the best damage dealing classes in the game, if played well, and is one of the most underrated PvE classes. 

If your group doesn't have crowd control, cycle between mobs. Flex Reavers should be able to move from monster to monster using Indigosnake (or after hitting 50, Leviathan) without getting much aggro.

Watch the group's downtime. If there is a lot of downtime, or if you have a power regeneration spell on, your debuffs might be worthwhile. If your group is pulling at a fast rate, however, you'll be better off saving your Power for lifetaps and your damage add.

Only use Wrack if you're trying to get aggro as quickly as possible. In other situations, it's not worth the power cost and the risk of breaking crowd control.

Use your debuffs to peel mobs off of casters and healers at the start of a fight. While Wrack would get the mob's attention also, it would also make it more difficult for your crowd control person to get the mob under control.

Always make sure you have Guard on a nearby tank and Protect on a caster or healer. (This should be obvious, right? Apparently it's not - and it's easy to forget, even for veterans.)

Don't use your damage add if there is a Wizard or Theurgist with a decent DA in the group. While yours adds more DPS than any other in the game, the 10 minute duration buffs are more effective over time; and you cannot conveniently use both, since yours over-writes other damage adds (aside from the Paladin's).

One of the best duos in the game is the Reaver and Paladin. Pre-50, the Paladin can hold aggro while the Reaver uses Indigosnake. The Lifetap proc on Indigo will greatly increase the damage done by the Reaver, and the Paladin should be able to hold aggro. In the event that the Reaver DOES get aggro, Boomslang should be used as a backup for Indigosnake. The Lifetap will keep the Reaver close to full health, and Boomslang will get the mob back on the Paladin quickly. Post-50, the Reaver should use Leviathan. A Reaver constantly using Leviathan can do very nearly as much damage as any caster, and has better defense, to boot. Again, Boomslang should be used as a backup for Leviathan, in case the mob turns on the Reaver.