When you commit to something, it is an unofficial contractual agreement that you are going to stick it out and follow through with whatever that thing is. Whether it be an extracurricular activity, a class you decided to take, a group you decided to join, or a relationship you decided to start, all (and more) require commitment.
One thing about commitment is that when it's given, it should be honored. You cannot say you're a member of a group or that you're in a relationship with someone else if you don't act like it. That's not what it's about. When you start something, you either need to keep up with it or you need to finish it. Wishy-washy is never the answer. Give it your all or don't even bother.
It is OKAY if you absolutely cannot go through with the thing you committed to before -- it happens to everyone. However, you need to realize when that point is and cut it off. For groups you join, you could definitely be hurting their progress if you join and then don't commit. For relationships, you could be hurting the person you decided to be with by not showing commitment to them.
The moral of the story is, commitment is a one and done deal. Some situations may be sticky but the point is that when you decide you're going to do something, you act like it. You give it your all and if you figure out it's just not for you, then you need to take it off of your plate. It will save you and the people involved a lot of time and energy.