Surprisingly little. I was concerned when we started Maddux, but it hasn't ended up being an issue.
There is a weekly girls group that involves all the girls across all grades (not sure if the pre-schoolers participate, but everyone else) that work on conversation skills. This is a highlight of the week for my daughter. Otherwise, her closest friends are boys.
We do things that expose her to girl groups -- for example, camp and girl scouts -- but I don't know how necessary that is.