Are you sure our children aren't twins?! Imaginary friends are completely normal and actually a good thing. My daughter has had the same imaginary best friend for over a year. This friend now has an entire family and they have mutual friends. It's really cute!
It doesn't sound like you have anything to worry about. You may have an event coordinator on your hands since she's already entertaining and hosting parties. :-) My daughter started doing little things (eating a cookie after I said no, not flushing the toilet, and ignoring me) and then blamed her friend. You may want to watch out for that one.
I nipped it quickly by asking questions about her friends and getting involved with some of their playtime. I would ask my daughter to describe her friends to me and just let my daughter take the lead during playtime. Eventually I learned that her BFF and family each had their own personality; which was a BIG help.
Later, when my daughter blamed her imaginary friend I would tell her that wasn't the person I saw doing whatever she did. I'd give her an opportunity to be honest; which she normally did. When she wasn't honest I just turned to her imaginary friend and explained that my DD was in trouble for lying and she'd have to go home now. After a few times of that, my daughter got the message.
I think imaginary friends are healthy. It promotes dialog and problem solving, just like real-life.