I'm new, so I don't really know how anything works, but from my very basic understanding, something like:
(xp gained) * (((lowest playerlevel*(remorts+1))/(highest playerlevel*(remorts+1)))
would give you the sort of sliding scale that was earlier suggested.
This isn't perfect, of course. I don't know if it reflects the benefit of remorts after GM level because, well, I don't know what the benefits of remorts after GM ARE.
We'll see how many times I can edit this post =) Edit count: 6
Another option, after reading Molly's post about grouping, would be
the above formula * ((5 - player's Tier) / ((highest player's Tier - player's Tier) + 1))
This would have the scaling effect from above, but would reward low-tier players for playing in groups, and give lesser rewards for mid-Tier players grouping.
This could potentially make multiclass characters advance significantly faster in groups than specialists, depending on how you counted tiers. This may or may not be a good thing, depending on which one you are =)
I should mention that all of this is based on a grand total of 7 hours playtime (half AFK), so this will have to be seasoned to taste.