I have a young friend (almost 19) who I have suspected may have it. I've known her for about a year and a half, and she is doing better, so I'm hopeful that this isn't it. She has been in therapy but I think she has stopped going, which is not so good - at the least she's depressed and has boundary issues. I've started a few threads about her before.
Reasons I thought she may have it:
-Didn't understand why it's not ok to follow people around, hang around their building uninvited when they're not home
-Jealous of my other friends, even mutual friends. Extremely needy in general, huge mood swings etc
-Showing signs of "wanting to be me" - ie. imitating my dress and hairstyle, glomming onto all my interests, try to work her way into all my friendships, etc etc
-Showed signs of "splitting" - this is a BPD specific term, I think, which I learned about when I was looking up some info - in brief it's like the BDP sufferer places the chosen person on a complete pedestal, OR views them as a horrible, awful person who hates them - no middle ground
-Calling me 25 times in a row when I wouldn't answer the phone...
Etc. I contemplated the idea that she had/has a crush on me as well, I will tell you that. But she definitely likes men and I think it is more BPD-type behaviour or "girl crush" behaviour. However, like I said she seems to be doing better - though she is still difficult to deal with a lot of the time and I have had to limit it severely. A lot of it comes down to that, I guess (but of course that is so circumstance dependent.) There are some pretty sad and scary stories in this thread already.
I have found that being extremely clear (even harsh) about boundaries helps; making myself a lot less available - ie. simply ignoring texts and phone calls or taking a lot longer to answer them, saying no and "being busy" (which I am, anyway!) A LOT, etc. However, I do care about her and have tried to be supportive - it is a serious balancing act. It wasn't working out a year ago and there was a lot of insanity. Although things are better now I keep a sharp eye on my feelings about the situation because I have realised there is a lot I can't deal with, and I wouldn't rule out her going really off the rails again, unfortunately.
Having reread this it doesn't sound nearly as severe as some of what has been described in this thread. I don't really know if there are degrees of it, or if some people just have boundary issues but don't have BPD. But I think at the very minimum, setting very very strong boundaries with any such behaviour is a must for your own sanity and self-preservation.