Here’s a good discussion on the issue (if the mods will allow it through):
http://www.chaburas.org/zimun3.html
It brings many opinions, most of which state that woman can make a zimun for themselves. There are even opinions (which we don’t pasken like) that a woman can lead a zimun with men in it (the Ritvah says this).
It seems the accepted practice is that woman can make a zimun if there are no men there, but they are not obligated to do so. Even if there are 10 women, they don’t say elokeinu in the zimun, as that is a dovor shebikdusha.