Need Suggestions for online sign up forms for group of volunteers

Hello,
I'm trying to help organize a group of volunteers.

Here's the problem. There are a few events over the next couple weeks. The people in charge send out an email to the group of volunteers asking for their help. Non of the volunteers know which events have more than enough people, and which events need more people, they all just pick which event they want to help with.

What I want:
Some kind of free online sign up system, that lets all the volunteers in the group see which events need more people.

Also, the volunteers have email addresses from a range of sources so I can't use anything that requires a specific account. I can't use yahoo groups cause it requires yahoo email account. Some of the volunteers are over 60years old and they will not sign up for a new email account. One even has an AOL email LOL.

Thanks