It depends upon the type of group. I see two types of groups. Natural groups are those which just come together such as a group of friends and Formed groups are those which are pulled together by some purpose, such as a group project for a class.
When it comes to natural group leadership positions, I'll only step in if nobody else does. I find I'm usually a very laissez-faire kind of leader and very little gets done when I'm in charge. I'm just not very comfortable directing people on what to do when it comes to natural groups. I'll do my best to meet the needs of everyone in a natural group and I'll be quick to yeild leadership to someone else if they want it. I'm very people focused when it comes to natural groups.
However, give me just a little bit of authority and that all changes. I see authority as responsibility and I have a tendency to feel very uncomfortable when I'm not in charge of a formed group. I'll create deadlines, keep everyone in the loop, and sometimes even become overly involved. I have no problem directing people when I'm in a position of authority and my focus is on the task and getting things done and done well. If other people don't pull their weight, then I'll do it for them and then I'll bitch them out about it.
I think authority really transforms an INFJ into an entirely different person. When I'm in a position of authority then I'll do as much work as I can, but when I'm not, then I'll look for ways to minimize my share of the work and sometimes just put things off.