Another solution Neveron has done are Battle Players. Essentially accounts can make secondary logons that have whatever access permissions they want, and another type that is tied to guilds/factions with the sam concept(permission based on guild rank).
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As for how it's handled in MOs they just use levels of scale, then gate even numbers into the battleground regardless of real numbers.

Thanks again, lithos.

But I don't think Neveron solution works in this case. Maybe FIFO and queueing (I probably misspelled ( I probably misspelled misspell too )) may help a bit.