You probably should do it on a first come first served basis, just distribute shares based on users ranks or activity points. Make sure to disallow newbie accounts created after today or if you want you could require at least junior member status for participation.
It's always a good idea to get coins out in as many hands as possible in the beginning, especially for smaller projects. People are more likely to spread the news and tweet about a coin if they already have some. Also it guarantees at least some activity if ever you get your coin on an exchange  Good luck!