I would not waste mana putting thorns on anyone but the tank and only if the tank can hold aggro consistently. Thorns is great if the mobs are hitting the person with the buff but not so great if the mobs are ping ponging around. Since you have a dedicated healer and crack in the group I would focus on dmg and keeping buffs up as well as snaring.
Here is what I do when I have a dedicated healer that isn't me:
-Keep thorns up only on tank (if aggro is consistent..skip this if it isn't).
-No need for HP buff with cleric in group but you could keep STR up on Rogue/SK, SoW up on everyone.
-Throw your DOT up early if mobs aren't dying too quick but skip this if they are.
-Nuke once or twice per mob depending on TTK speed of mobs and/or mana availability. Nuke late in the fight to avoid pulling aggro.
-Snare mobs at 30%-ish health if they are fleeing mobs that could present an issue when they run. Some mobs never flee like undead so don't waste your mana. Other areas like outdoors it may not matter much if they flee a bit.
-Never run yourself completely OOM with nukes/buffs if possible. I always try to keep about 30% mana to spot heal and help out the cleric if things go south or the cleric needs to get back some mana (or later on you need to evac). IMO It's generally better to space out your dmg across all mobs than go nuts on any one mob and drain your mana.
-Use root to help chanter CC adds or when/if chanter dies.
-When not doing any of these things meditate to keep mana up. Also, make sure that chanter is keeping Breeze on you.
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To facilitate this type of play in an efficient manner I would highly recommend getting a program called GINA. It reads the log file and gives you some buff timer overlays to help keep track of when buffs are fading. This will help a ton..especially with short duration buffs like thorns and will allow you to keep all buffs up 100% of the time and to stagger buffs to be more mana efficient. You also won't have to waste mana buffing yourself with things like STR or thorns just to keep a timer of the buff on someone else. People will love you when you are efficient at keeping their buffs up without having to ask every 5 minutes for SoW or whatever. This is even more true with a class like chanter so get your chanter to use it too for hassle free haste/crack.
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Also, since you have a bit of time until you have to specialize (lvl 30) I would pay close attention to what is most beneficial to the group (nukes or healing/utility) and then specialize in that at 30. I think in general alteration is a better spec for Druid because you get the most bang for the buck (healing, buffs, ports) but if you really are consistently acting as DPS for your group and you mostly level with that group it might be worthwhile to spec in Evocation because druid nukes are pretty good but not very mana efficient so getting cheaper nukes (and some dots) could be beneficial for you in that case.