If it is the first time you have met someone, it is perfectly acceptable to go up to them before the meeting and introduce yourself. It is also a good idea for your boss to introduce you when he starts bringing you to meetings and making sure that everyone introduces themselves to you. Since he didn't, he probably thought that you knew who all the people were which means they are probably relatively senior managers or people who work with your department frequently or he didn't know who they were either and didn't want to get embarrassed.
So now it is more embarassing that you don't know why they are and more awkward to ask. So first let this be a lesson to you, you should have spoken up at the first meeting.
Now you have several choices. First if there is someone else fairly junior at the meeting that you know (especially an admin assistant who is taking notes and almost certainly knows eveyone), you can throw yourself on their mercy and ask who everyone is.
It is best not to do this to anyone senior to you (As it is not terribly politically imporessive) unless you know them very well with the exception of your boss. As @jdb1a1 says, you can ask him after the meeting who was the guy who suggested this or who was opposed to that. Or when someone is talking, you can write him a note (if you are sitting next to him and if he isn't the kind who will be annoyed) and ask him who the person is.
Another way to figure out who is who is through meeting minutes if you have them. Take your own notes and describe the person who is saying something major and then compare those notes to what the meeting minutes say and you may find out who is who. Sometimes you will hear them called by their first name, then you can try to match that up to the who name on the meeting request.
Of course, you may know who all the players are from the meeting request or minutes but not associate faces to names. So take your list of names and find their offices and that way you can see who is who. If you see one of the people in the hall, you can go up to them (if the person doesn't look too busy or unapproachable or is standing outside a very senior manager's office) and say something like, "I'm Joe Smith. I saw you in the XYZ meeting and didn't catch your name." Then make some small talk about the meeting.