As far as I understand, all new players/first deposits get six invites (3 times per week, for two weeks). You can only play those specific times you get invited to (that's all 3 freerolls in both of the two weeks after your first week, they're on tues, thurs and sat and the invites come for all new players from the previous week, so it doesn't matter if you joined on a wednesday or on a sunday - your first new player qualifier will be on the next tuesday).
I've done 4 so far, two more to go, very few people (like under 60, I think) were in each, and it was ridiculously easy to make it into the money. (Except for that last time, when I was a bit tipsy and really didn't care and pay any attention). If I'm not remembering it wrong, on one occasion we all actually started out already in the money, because the min. number of players is 25 and top 25 get $11 each in tournament credits. The tourney cred work like money for entering any scheduled tournaments - you're not committed to to playing in the 100K series with it, I think (I've certainly been using it for other tournaments).
Definitely recommend doing those.