Google has set up an environment that makes it pretty easy for people to transfer between groups, and even between sites. You have to be a grown-up about it, of course, and not ditch your team during some critical crunch time. But provided you exercise mature judgment in the matter, you can pretty much switch teams whenever you want, and it doesn't cause the company to grind to a halt, as such a policy would at other places I've worked or visited.
