Note that a very common scenario where this happens is when people who should be introducing you as a matter of basic courtesy, sometimes don't because they don't remember the other person's name in the first place, or they find the whole 'introductions' thing awkward themselves (e.g. "what do I introduce them as?", etc).
Typically they are embarrassed to admit this, and are also secretly hoping you'd go ahead and introduce yourself independently first, so that they can remember the other person's name too / avoid the awkwardness, etc (at which point they can also go, "oh yes, sorry, I forgot to introduce you two, hahah, silly me!").
So don't assume that if you're not properly introduced to other members of the group that it is because there was no need to. Just go over and say "hello, my name is X" and remove the awkwardness for everyone involved. If you don't particularly feel the need to go through small talk after that, or can't think of a topic to talk about, there's no need for that either. Just say "nice to meet you" and then go back to doing whatever it was you were doing with your friends, and that's totally fine*. That way, even if something comes up later you can talk to them about, you can now do so without being embarrassed that you haven't met them.
* obviously, having said that, if you're not planning to hold small talk with someone, obviously it's best to get the "nice to meet you" bit as soon as you're introduced to that group setting, otherwise it becomes awkward if you walk up to someone about an hour in, and then walk straight off again.