Probably you have seen some men who they are along with women in a club, bar etc.
This is not bad, but many people are afraid to approach a woman if there is a man in her with her.
Most of them are just "orbiters" and are men that are friends, colleagues or brothers with these girls. The first thing you can do is to watch if the girl touches or kisses any man from this group before you approach this girl.
I believe you can understand very easy if there is any sexual connection between a man and a woman. If you don't, go learn the basics first.
A few days ago, I approached a girl who was in a mixed set. I approached her only after I watched the people there and I saw that they are not having any sexual connection. The man was gay and he was a friend of my target. I couldn't even imagine this if I didn't approach them. So, you never know how do they know each other.
So, don't hesitate to approach a mixed group, but do it only if you are sure about their relationship.