of course you would do it from life if you want to do it right. the only way to learn properly is direct observation. ask your friends if you can sketch them, just a quick drawing, to get the forms. later you can imagine and change all that you need, since you will already have a direct visual copy of the forms you need. the life though that they need is best taken from life; and full works with your friend in front of you is best. if your friends are poor sports (and i dont see why they should be, ive never had trouble asking people to pose) then you can use a mirror and do yourself. last resort is to take a picture of yourself or someone that you know well, because at least then your mind is familiar with them enough to put something real behind the photograph.
http://img157.imageshack.us/img157/9987/facerefsheetax0.gifthis is a quick reference sheet that i made for myself about 2 years ago. its not as good as drawing from life, but creating yourself a basic sheet with several genaric positions can be useful in framing out the basics of a portrait. Also, when using pictures of friends, one good idea is to not take a picture of them posion, but to find a picture of them that really shows them. spntaneous pictures always make for better references because they retain more life. here are some pictures of friends that ive used more than once because they both have a lot of life
http://img145.imageshack.us/img145/6241/courtneysmallnm0.pnghttp://img301.imageshack.us/img301/982/andrewkf5.jpgif you must use pictures, try looking for similar ones of your friends. If you dont take that many pictures, Myspace can occasionally be a good source, though often its a bit too posed. worth a look though for sure.
the point of all this is that you should have something to go on besides just a photograph. a memory, emotion, a bond of some sort. this will always help you in drawing effective characters. strangers, even celebrity strangers, almost never yield drawings as good as llife experience and observation.