I got an iPhone4 last week and was about to call my friend yesterday when I noticed a name next to hers in my contacts that I didn't recognise. I scrolled through the rest of them and there was a few new names, all email addresses. Turns out they're my boyfriend's friends but why would they randomly add to my contacts? I thought maybe he's signed into MSN or Hotmail on my phone but he said he hasn't and I don't think he has either.

Even if he had, why would it add them to my Contacts book? I've deleted them obviously but I still don't understand how they got there in the first place

Does your boyfriend use Facebook on your phone? It may have imported contact details from there.

He hasn't used anything as far as I'm aware and he said he hasn't and that he wouldn't know how to. It just seems to be a bit of a privacy issue to me if it's adding peoples contact details without even asking me.

It hasn't added them randomly. Someone, either you or your boyfriend, has explicitly authorised it to import contacts from elsewhere. It may have been done unwittingy, but it's nevertheless been done. The phone doesn't just add contacts all by itself.

Think about it, how would the phone know how to get his contacts all by itself?

If your iPhone was at any time plugged into his computer, it would have synchronised his contacts (and other things) to your phone.

But again, not without iTunes installed and not without permission. It hasn't done it randomly by itself as the OP is suggesting.