Edit: seriously, unless you plan to somehow monetize it, having Google do your sysadmin work, pay for your hosting, and present a well tuned web/email interface at no cost is a no-brainer. It's early enough in the project you could ask people to switch over.
At the same time though, if you host it yourself you can switch out the backend or interface whenever you want with no inconvenience to users unless going in to or out of a locked-in system like google.
Hosting is cheap, and someone running a "weekend hacker" group probably already has all the hosting worries taken care of for other projects. Besides, running things is fun.
Edit: seriously, unless you plan to somehow monetize it, having Google do your sysadmin work, pay for your hosting, and present a well tuned web/email interface at no cost is a no-brainer. It's early enough in the project you could ask people to switch over.