Social proof can't explain it but I've known very few individuals who have the guts to approach a party of mixed sexes (e.g. who are couples - when it's all women - you can assume they're just socializing, not dating (usually)). Actually, it doesn't matter whether it's all women or mixed - still intimidating.

The times I've done it / the first twenty seconds after you have broached their airspace is among the most scary and exhilirating experiences. The more women, the better. Do not hover!

For bonus points, sidle over and drag a chair loudly from one side of a room to where they are and sit down (presuming they are seated, you have most likely pulled off the icebreaker that needs no icebreaker)