If you want to join in, just start talking at whatever seems the most appropriate spot and don't take it personally when they don't all stop and listen to you. You have to talk about female stuff and keep your male ego in check when they talk over you. Stop and listen to them. They are talking over each other. This the the hardest thing for men to deal with. Usually one of the more empathetic women will stop and try to listen to you. But, it's just so awkward because you feel like you are just keeping her out of the conversation and she really isn't interested in what you are saying. We've all heard that women are better at multi-tasking than men. This isn't folklore and it isn't just training. Women physiologically have a larger corpus colosum than men do. This is the network in the center of the brain that keeps the two hemispheres communicating.
Joining into a female conversation and holding your own is one of the most difficult things a man can attempt. But, if you find yourself in a situation where it would be appropriate to try, you should.