Women and Zimun

(7) Women, slaves and minors are not eligible for the quorum required for the introductory formula. But they recite it when they are by themselves. However, to prevent licentiousness, women, slaves and minors should not be formed into a joint company for Grace. But women may form a quorum for themselves, and so may slaves. In either case they do not use the name of God in the introductory formula. An androgyne recites the introductory formula if the entire company is of his class, but not if the other members are normal men and women. One whose sex cannot be determined never says the introductory formula. A minor if he is only seven or eight years of age, provided he understands Who it is that is addressed in benediction, may be counted in the quorum, whether it consists of three or ten persons. A non-Israelite is not to be counted.

"Women make a zimun for themselves, slaves may make a zimun for themselves, they may not join together to form a zimun."

Q: This implies Women are obligated to make a zimun amongst themselves, yet this is not our custom...?

A: "Women make a zimun for themselves [if they want to]."

This is implied immediately afterwards in the Beraisa when it says "If [women and slaves] want to join to form a Zimun, they may not."

The Beraisa is using a language of Rishus, not Chiyuv.

He further reasons that if they are obligated to say Birchat HaMazon, either on a d'oraita or a d'rabbanan level, then they should certainly be obligated in zimun as well.