Rather than expand the server, why not start kicking people out? Quite a number of people who in the PBE are toxic players. Why are they allowed to abuse the server for free goodies and advanced playtest?
The solution would be to start linking the PBE account to our normal accounts along with the following restrictions:
1) Must be level 30 on main account
2) Getting banned on main account bans you from PBE for that same amount of time
3) Harsher punishments from PBE. Banning from PBE won't affect main account, but punishment is greater and if kicked out from PBE, the account stays linked to main account, preventing you from reusing that account to be linked to a new PBE account.
These steps would lower the amount of people in PBE because 1) Less accounts will be made to clutter up the server 2) The same toxic people won't be jumping onto another account to enter the PBE 3) They are held accountable for their actions because if they are toxic in PBE, chances are, they are toxic in their normal accounts as well.
Growth in PBE population is inevitable, but implementing these would help reduce the population somewhat and makes for a more quality environment to test for bugs and glitches.