(And still get experience from kills.) You can always group with characters within five levels of you. Otherwise, the higher level character can group with any character that is 2/3 of his level (rounded up). This is subject to a maximum difference of 30 levels.
When AA experience was adjusted, the result was that a character will only get AA experience from mobs that are better than grey to the highest character in the group. Any experience that would be voided by this rule may be passed on to level experience, even if your character is set to 100% AA.