According to this answer, three women who have eaten together may make a zimmun. The usual formulation of the zimmun, "רבותי נברך", does not make sense for women. What formulation should women use when making a zimmun?
Dan's answer addresses the salutation.
The invitation to bless, however, might also do well to be adjusted. While in modern Hebrew (and, indeed, for a very long time, historically) the feminine, plural, first person call to bless is the same as the masculine (נברך). However, a more archaic form of the word (and thus, perhaps, a more correspondingly appropriate feminine equivalent to the standard men's Zimmun) is נברכנה.
I would suggest that an appropriate formulation (though by no means the only valid formulation) would be גברותי נברכנה.
That's assuming you stick with Hebrew, of course.
Rabbi Dr. Ari Zivotofsky, in his article on women's zimmun (published in the OU's Jewish Action Magazine, Fall 5760/1999 issue (link)), suggests gevirotai or chaverotai.