There are many roles a bard can take in a group. As a bard gets into the mid to upper 20s they start to gain songs which are extremely useful in certain situations. Up until 18th level, combat usually involves Anthem de Arms, Hymn of Restoration, and Brusco's Boastful Bellow. But at level 27, there are many choices a bard can make on what instruments to play and songs to sing, or even whether the bard should pull out the weapons (rarely necessary). Here are a couple of "roles" that Cleo has put herself into over the last few days:

Goal: Do as much damage as possible.
Songs: Chords of Dissonance, Denon's Disruptive Dischord, Psalm of Warmth.
Instruments: String and Brass

I use this combo when I'm in a group with a cleric and 2 tanks. The cleric can keep the tanks healed, the tanks can keep the monster busy and I'll be the designated nuker. I'm not sure how much damage my songs are doing anymore, but I'd guess 14 from chords, and 22 from dischord, plus 5 a hit from warmth. Dischord also lowers AC, which I equate to the Tanks version of Tashan (enchanter's spell that drops magic resistance).

You need to swap instruments with this combo. It's mind-numbing at times, but the benefits are great. Cyclops fall easy with 2 tanks a bard and a cleric at level 27.

Goal: Turn the tanks into deathmachines
Songs: Anthem de Arms or Chant of Battle or Selo's Consonant Chain, Hymn of Restoration, Psalm of Warmth.
Instruments: String and maybe Drum

This combo works best in two situations, when you are in a party with 3 or more tanks, or when you are fighting creatures which resist your songs. If it is a party full of tanks, I use Chain, Restoration, and Warmth. The Chain will slow down the critter's attacks and keep him from running, meanwhile you are increasing the regen rate of the tanks and putting a damage shield up around them.

If the group is fighting deep red critters, then I drop Consonant Chain for Anthem de Arms or Chant of Battle. Here you have another decision. Does the party need to swing their swords a little faster, or hit more accurately. Chant of Battle with a drum will buff Str and Dex higher than Anthem de Arms. So if the party needs more accuracy (having trouble hitting the critter) then Chant is more useful.

Goal: Caster vs. Caster support
Songs: Guardian Rhythms, Elemental Rhythms, Chant of Battle
Instruments: Drum

Until level 32, it's not likely you'll end up in a party of all casters. Sometimes, they'll find you beneficial. When you buff their magic resistance to around 150, they tend to take less damage from other casters. Meanwhile you're buffing their dex up for additional accuracy and spell interruption recovery.

Goal: Take Damage
Songs: Selo's Consonant Chain, Psalm of Warmth, Solon's Siren Song
Weapons: Fast, light weight ones, you don't get double attack, your stamina is low. Use fine steel daggers if you must. I use a Bladed Thulian Claw (5/19) and a Dragoon's Dirk (6/23).

You're trying to keep the monster occupied, perhaps because the tanks are heavily injured, or because you know you have enough hps to tank it. The goal is to keep the monsters on top of you. Siren Song is the best choice for taunting, keep Psalm of Warmth running and weapons swinging and you'll be sure to keep taking damage (ick). Of course, until you get Siren Song, Consonant Chain is your best bet. At level 23, 3 singings of this song would pull anything on to me. By level 27 I can't taunt squat with it (which is nice in my opinion).

I used this once, and died for it, but because the healers were able to get enough mana for a heal for our tank while I finished off one of the nasties, 3 out of 4 lived, and a cleric resurrected me. If we weren't in a bad spot in the dungeon, I would have pulled out the horn and used Appalling Screech. As a bard, you have to know when NOT to use a song more than you need to know when to use a song.

I hope this helps all the bards out there,