Basically, if the 5's are directly linked (say Account B and C, or D and E in the chain).
They're grouped together and in the same area.
Obviously they're within 4 levels of each other, if they're both 5th.
As far as I know, nothing is grey to a level 5, so mobs/quests are not grey to either.

With or without an 80th in the mix, whether the 80 is in the RAF chain or not, they get RAF experience.

Assuming the accounts are say BC.
And the chain is ABCDE.
The 80 could be on A, D or E, and they'll get RAF because BC is a direct link.