Is that 90,000 active members or 90,000 including inactive members (i.e. no valid accounts) or 90,000 active accounts with only 20,000 actual people? I know a few people who have multiple accounts (though I do not). I'm not sure how you could even tell, quite honestly. In any case, you'll be inviting a lot of registrations from people who already have accounts, to just have those accounts become unused. If that's your goal....