Appropriateness with Her Friends
For years you've been able to pick up and drop off her friends, host play times, and take them all for lunch or a walk at the beach. Now, things are shifting. As girls grow into young women, their perception of all men and actions around all men change. While they once may have thought of you only as Suzie's daddy, now it changes.
Removing the Sexual Undertones
As shocking as it is to see your own daughter develop, it can be disarming to see her friends develop as well. Accepting this gracefully takes care and forethought for a dad. Dads don't want to accidentally wander down that road that Lester did in the film American Beauty. Luring and lusting is never okay for a dad around his daughter's friends. You'll want to limit, if not completely avoid, any comments on how girls are growing or how they look. Even the most innocent (from your point of view) comments can be misconstrued. Avoiding them is your best bet.
You will also want to avoid getting too personal with your daughter about her friends. If she wants to talk to you about friendships and issues, that's one thing. If you are the only parent and your daughter needs to talk about this, be sure to let her know it is all in complete confidence and you'll never give a hint to knowing.
Appropriate Attire
Maybe when they were small and her friends were over you'd walk through the house in the short-shorts you wear while doing chores and not much else. If you never stopped it before, you need to stop it now. Cover up around young women; not because you've anything to worry about or be ashamed of, but to avoid any misconstrued reactions or embarrassing moments for your daughter and her friends.
Other than at the pool, at least a T-shirt at all times is a good rule. And never, ever, come out of the shower in a towel while friends are hanging out or sleeping over. You may also want to be sure to wear pajama bottoms during sleepovers, even if you usually sleep in the buff. Should an emergency occur, you'll want to be appropriate without having to think about it in the rush of the moment.

