The only qualification is that you have to have a less than '2' handicap..
she received a 'sponsor's exemption'. Those are the 4 or 5 spots handed out to whoever the sponsor wants to. Often they go to good local college players or an older professional with some sort of local tie-in -- people like that. Annika is not costing anyone a spot in the tournament who would be considered a regular tour player.
Sorenstam did not take the place of a card holding player. She did not sue the Tour or Sponsor to force them to let her play. She was invited by the people who pay for the tournament and assume all the risk.
For each PGA tournament, the sponsor gets a limited number of exemptions that they can distribute. Some of them must go to tour players not exempt from qualifying, but the sponsor can give a few to pretty much any golfer with a 2 handicap or better. Annika was given one of these exemptions. In short, she was invited in a proper manner.
It will be interesting to see what the gap is between the best men and women.