what about a rank buff, when you join behest, your rank is capped down to a specific rank, and simply have a requirement to join.
Example: XxXSephorossXxX is rank 12, he tries to join a rank 20 behest at camp horizon, and is told by the npc, he is not strong enough. XxXCroudXxX is rank 17, he tries to join a behest at the same camp, being closer to the camp's level, he is permitted to join in.
Example 2: Zeedan is rank 40, he tries to join the same behest, due to his rank far exeeding the camp's requirement, he is temporarily reduced in rank to Rank 20, to allow him to join in the behest, but not impact those who are the appropriate level. This supports large behests at all times, not just when there are plenty of lower ranked players there.