Bridesmaid Selection Question

Have you ever picked someone to be in your bridal party that you weren't super close with, but considered a close friend?

I know that's kind of an oxy-moron, but long story short - our Groomsmen have been both mine and my Fiance's best friends since we were in Elementary School. They're so close that we're not picking a Best Man. We're having three Best Men, plus my brother.

On my side, I only have three girls, so we're uneven - three girls and four guys, which I'm okay with.

My dilemma is this: One of our Groomsmen is engaged to a great girl that we've known for almost equally as long. Seriously, they've been dating like 7 years. I adore her and I expect that we'll be life-long friends. The only thing is, she's in dental school, so we hardly see her. She comes out every once in a while, but I just wish we got to see her more sometimes.

Anyway, I was thinking of asking her to be a Bridesmaid, since we consider her equally as close of a friend as her Fiance, but there are two reasons why I'm unsure:
1) She's in Dental school and is very busy
2) She's planning her own Wedding which is tentatively scheduled to happen 3-4 months after ours in Disney World.

Basically, I'm afraid that I'll regret NOT asking her, because I do think we'll be close friends for the rest of our lives. But I'm also afraid that she may feel obligated to say yes if I ask her, which I don't want either.

I don't have any sisters, so I guess this makes choosing harder for me. I also want to choose for the right reasons.

What would you do in my situation? Also, what might you say if you were in her shoes?

I say ask her!! she can always decline if she doesn't feel she is up to the task. if you truly think you will regret it and she is a lifelong friend then i say ask her you can always express your concerns to her and tell her i know you're soo busy so i understand if you cant make the commitment but i wanted to extend the offer since i consider you a close friend