The key action on a page is selecting "I'm in" (meaning "I'm committed", which can also be deselected). The logical choice of controls is a checkbox:
[ ] I'm in
I'd like to give this action elevated prominence and integration with other actions by displaying it as a large button with an icon:
-------------- | ICON | | | | [ ] I'm in | --------------
My concern is that first-time users might miss the unchecked state and assume a large box saying "I'm in" indicated they're already "in".
What visual cues could be used to leave no doubt the call to action is to click the button to set the state? I'm open to having the use of the combined button and checkbox challenged if alternative ways of giving prominence are suggested.
[EDIT] As some of the suggestions have highlighted, the unchecked state means "interested" rather than be as strong as "I'm out". Interesting how this impacts some of the options.