What if we allowed people to toggle the experience bonus you get from being in a group to the inverse of it?

If I want to rank fast, a group is optimal still, being I don't choose to nerf the experience bonus.

If I want to perfect skills, I can turn the experience bonus into a penalty, group with people, killing tougher mobs since I'm in a group, but not getting more exp, because I've applied the toggle to reduce exp. Thus allowing more time at each rank to work on skills, without forcing me to avoiding grouping in order to buy that time.

This means grouping becomes viable no matter what your objectives. And involves just a flag to say to divide exp instead of multiplying it, when grouped.