We mix it! My boyfriend's best friend is my best friend's boyfriend (confusingly worded!) so we see eachother as a foursome sometimes. As a group of girls, we sort of used to mix with this group of boys (who are my boyfriends group of friends) occasionally, used to go out for a drink with them in the same places etc, so we do know eachother and they all get on well. If I'm doing something with my friends, my boyfriend isn't automatically invited though, and neither am I when he's invited with his friends - we both appreciate we need our own girl/boy time with our own friends as our own small group. We do often go out with eachother and I know and get on with most of his friends and he gets on well with mine so it's nice. Apart from one friend who doesn't like him (her problem) so I try to keep them separate but equal which is difficult especially when said friend makes things difficult. We spend a lot of time alone too and go out just us, but it's nice that we can both go out with eachother and not feel torn between friends and boy/girlfriend.
It's tough trying to balance boyfriend, friends, work/school, alone time and family though!