When not in group, you ought to get some XP, and a chance at loot and quest completion, if you were not first hit and have contributed, say, more than 20% overall damage done.
In other words, if you “tap” the target first then you have full rights.
If you were not the first, then you still get quest rewards etc if you have done 20% damage done.
The above proposed system does have faults, but is much better than being left out completely and have to camp for hours.