Unlock one level at a time when you reach the profit zone.
Some guilds I've been a member of had a thread specifically for GBs in the profit zone. You'd ask for 2nd and 1st and only unlock the level when donors for both spots were available and waiting. Others just asked in the regular 1.90 thread or in the guild's main thread.
Otherwise, secure one spot at a time - secure 1st and 2nd and ask for donations. Once filled, secure 3rd and ask again. Then 4th and when it's filled, secure and ask for 5th. This way, no one can donate less than 1.90 to lock themselves in a spot. Avoid the temptation to secure all spots at once. I learned it the hard way.