It does sound as if she has learnt this behavior maybe from her other friends and is curious, it could be as simple as that ...children are curious about their bodies, however it may be a good idea to do more supervision and be aware of this, dont let the friends go upstiars now you know they are doing this, there is usually one instigator possibly an older,child, it could be she has seen this behavior and is copying it, you could mention it also to the friends Mom, dont leave them alone together ..