It is so hard to have to be put into a situation like that. Just try and do your best. A lot of team building exercises are pretty structured and usually there is someone who is more than willing to be a leader of sorts so that may help a bit. If they hit on something you are comfortable at all with I would pounce on that bit so that they don't think they have to try too hard to encourage you to participate in other ways that may make things worse. Good luck. Let us know how it goes.