[Lazortag]

Rangers can tank just fine from 1-60. They hold aggro well and their mitigation/hp is fine. Whether you can tank in groups isn't a matter of how good your hp and mitigation is relative to conventional tanks (wars, sk's, pallies), but instead whether your hp and mitigation is good enough for pulls to still be efficient and for exp to be reasonably fast. Here's a list of classes that usually satisfy this condition:

Pally, Monk, SK, Warrior, Ranger, Bard, Shaman.

Usually if you suggest a ranger, bard or shaman for a tank in an exp group people will raise their eyebrows, but I've pulled for groups with the above classes as tanks in zones like Charasis and the exp was still pretty fast (definitely faster than the average pug). Be open minded and don't just deny someone a group invite because they aren't an SK, Pally or Warrior.

edit: I agree with what people say above though that the ranger should be wearing tank gear though.