Pheenix, I have the same problem, but for different reasons.
It's usually not that I don't have anything to say. In fact, I often want to say something but I just can't keep up! I look for a break in the conversation, but if it's a group of chatty people, I have a hard time finding a place to interject. Or I might get out a syllable, but then someone starts talking, or I end up accidentally talking over someone else so I stop. I just feel it would be rude to mow over other people in conversation, so I have a hard time in a group of people conversing together.
In your case though, it sounds like you just need to not worry about it, if you really don't have anything to say. If people aren't talking about something that interests you, or you really don't have anything to add, I don't see any reason to force it. And if this is happening all the time, then you probably aren't hanging out with the right people for you!