Hmm... my suggestion would be to /ooc Hey, we are looking for a tank in our group. Anyone want in?
Tanks will make your life easier. You may get lucky and meet a good player to join you guys. If not, can i suggest healer/mage/scout.
For healer, any will do. Druid types seem to do the most damage, Shamans get the best debuffs, and templars get plate armor. Hard to pick. Templar would likely tank the best with the plate armor.
For caster, I suggest a pet caster or an Enchanter. Pet classes can let the pet tank, which would be useful considering you have no tank. Illusionist may be a great idea, too. You get a great pet that mezzes and stifles, and scales to your level. Plus, Illusionists can mez, which makes them the best at handling adds. Problem is their nukes are weak, but they have good dots.
Also, their power regen will help your overworked healer.
For scout, any will work. Bards have great buffs, but assasian/ranger will do killer DPS. Honestly, just go with what is the most fun.
for