I'm sure everyone has been in this situation at one point or another in their life, but I'm wondering what you guys do when your friend talks to someone you don't know well or don't know at all.
For me it's kind of (Actually, Really) awkward because I'm not too open to random strangers, and most of the time my friends don't take the initiative to introduce us so I'm left their to try to try to act as inconspicuous as possible for a awhile or pretend like I have something to do (looking at my phone is one of 'em) as to not seem weird (which ironically mostly backfires).
I know this isn't a serious issue, but I'm just curious how you guys deal with this type of situation.

I do the same exact thing as you XD I get nervous around new people so I try to stay behind my friend and normally pick up my phone. Lately though, I have been trying to at least say 'Hi' to the strangers. It makes things just a little less awkward :P .

On how i deal with them, i just be cool with them and if they talk to me i answer back...

I usually get nervous as well and say hi if they talk to me or I pretend I have something important on my phone or ipod

This one is hard... I usually let them have their time, but if my friend left me for that friend.... Consider my trust utterly crushed.

I am exactly the same way. This usually happens to me when I am out at a mall or some place public. Most of the time, I in the background, announce I will be in the next store, food court, ect. And generally just go somewhere else. It is next to impossible for me to make conversation so I try not to interfere. If there is no alternatives for me I'll mostly act uninterested and just sit down somewhere or lean against a wall and chill on my phone. Some people have told me to try and chime into the conversation if I can and introductions will come later... having tried that. I can honestly say it does work, but most of the time there is still that giant invisible wall as introductions never happen and you end up conversing with a stranger. . .

I usually say hi and try to make a conversation or change the conversation everyone can join in. If the person is a little bitch behind my friends back, i'm not afraid to start and argument. I really treat them the way they treat me. If they're silent, i may attempt one or two conversations but beyond that, i dont go out of my way.