Not knowing your daughter, this is a pretty generalized and probably inadequate response, but I remember well being a 7-year-old girl. My parents moved and I switched schools, that year. I also have a 7-year-old son, who's still working on finding his friend-making groove. He went to a small, cooperative preschool and kindergarten, where even a child with no social skills would still wind up with lots of play-dates, because of the friendships between the moms. It's not quite the same at his big, public elementary school, and sometimes it's hard watching him find his way, or wondering why I seem to host play-dates more than he seems to be invited to them, right now.
I have no recollection of being insecure, at 7, about whether or not I had enough friends. I did make them, at my new school (I am friends, to this day, with 2 of the 3 best friends I ended up making there), but like your daughter, I didn't mind going from one group to another on the playground; and I was sensitive to other kids being left out of groups and might leave mine, to play with them. I don't recall feeling left out...so, either I really didn't feel that way; or I did, but those feelings were not significant enough to stick in my mind, long-term. When I found kids I really connected with, I bonded with them just fine. But my 3 best friends were each very different, and did not hang out with each other: one was an outspoken, intellectual girl; another was a bubbly, popular girl; and the third was a "boy next door"-type (grew up into an Eagle Scout...). I have always liked and appreciated a wide variety of personality types. Accordingly, I've never really been at the center of some homogeneous clique where I clearly "fit in", but I've always had satisfying friendships and felt accepted and welcomed by several different groups.
If your daughter turns out this way, I can see how you might have periods of worrying whether she's being/feeling left out, or whether she needs work on her social skills; but if she doesn't seem unhappy, then maybe she just needs time to cultivate bonds with people she really connects with. I think it would be important to make sure she feels very loved and grounded in your family, and treasured for exactly who she is; and also to facilitate her participating in some extra-curriculars, where she can meet other kids, and/or have a forum besides class/recess to interact with some of the kids she knows from school.
Of course, there's the concept that children enter the age of reason, around 7. It does seem easier for toddlers and preschoolers to socialize with just any kid who happens to be available - since the chief motivator is often what they're doing or playing with. By 7, it starts to be more important that friends "click" with each other on a personal level. There's more thought and chemistry involved. It can wind up being a real strength for your daughter, to be comfortable finding her own social path, and not to be obsessed with how she fits in, or trying to be just like all the other girls.