I'm going to call the person being boosted person A.
Lets say you have person A and two more people, level 80s, person B and C.
Person A stands outside of RFC (For example)
Note: Noone is in a group at this point.
Person B and Person C go into separate instances and pull the WHOLE thing (or at least most of it) and take them back to the entrance.
Person B then invites person A to a group.
Person A enters the instance with Person B.
Person B kills all the mobs.
Person A gets a full runs worth of exp (running xp, not full, but still good)
Person A then leaves the group and does the same thing with Person C.
Person A has just received exp for two runs worth of RFC and loot if he/she bothered looting the mobs.
Note: You have all the mobs waiting at the entrance when you have Person A running into the instances for the kills.
Now, you could do this with more people than two running.
NOTE: You will be limited by the "You have entered too many instances recently" message
Please read the whole thing before commenting, and tell me if I need to clarify.