I covered my view on group prayers (saying grace, etc.) in an earlier post, but there’s one aspect I left out — what if they want you to join hands? Does that change the equation? To me, it doesn’t. If you’ve already decided that being polite is more important than standing on principle (which is, of course, a whole other issue), then joining hands isn’t any more insincere than just bowing your head.
A Christian prayer uttered in a group holding hands is just like any other prayer — it’s not a frickin’ seance. Nobody’s pretending that there’s some magical “circle of faith”-type power operating and that you’re somehow violating their pseudo-magic if you’re a non-believer. It’s just a gesture of fellowship.
The other people don’t believe their prayer won’t “work” if someone in the circle isn’t a believer — or if they do, and they find out, and they give you a hard time about it (i.e. if they’ve stopped being polite and started being rude, thus ruining any chance of maintaining a polite atmosphere at the gathering), just tell them that if it was so all-fired important to them, they should have taken steps to determine that everyone was a believer, rather than arrogantly assuming it.
You’re not being any more insincere by holding hands than you’re already being by bowing your head and not making trouble. So don’t sweat it.