Only people who are members of the group (obviously) and people who don't have the game yet can join.
And the game has to be added to the library by the person who wins it.
For those who aren't members yet on steamgifts.com, you can join if the total price value of your owned games exceeds a certain amount. I think is it 100$ or so. Giveaway starts in 5 hours and ends in 5 days.