Here are a few ways you can try:
See if the poker room you are working with will accept an allowed list for those players account info (email or screen name) you send to them a day or two prior to the game then have your members post or PM info for you to put together.
I have a group of players who consistantly play my buy in games and support the forum by using our links to sign up to a new site or those who deposit with our links, I call them VIP members, these are players that are tried and true members and onloy they get the password.
I also run added buys and only those who play the buys get the password either emailed or PM'd to them the day prior to the game.
And... When I do have the dreaded "open forum freeroll" that anyone can join as long as they are a member, I post the password an hour or two prior to the game. The VIP's get the password about a 5 days in advance. (Hence another perk to being a VIP).
We all know many times a room may set the minimum player threshold high and if we only go with those that are under us, it is hard to get the minimum.
Just another reason why I abhore freerolls.
Hope this helps.
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"People who are unable to motivate themselves must be content with mediocrity." ~Andrew Carnegie~