1. Spam.  The world wide web is full of spiders.  No, they're not the eight-legged variety.  Internet spiders are robot programs that search through groups, read through member lists and message boards and grab any email address that happens to be showing.  The email lists are sold to spammers who then flood your email inbox with all sorts of junk mail you really don't want to see.
2. Spoofing.  Someone who doesn't like you can use your email address to spoof your identity in a group and send the group emails that look like they came from you, but actually didn't.  They can really damage your reputation online if you aren't careful. 
3. Passport Security.  Knowing your email address is half-way for someone to hack into your account.  People usually aren't as careful about passwords as they really should be, and quite often it's easy to make a guess at what someone's password is.  Once they have that, they can create all sorts of havoc in your name in your groups.
4. Stalking.  A stalker can add you to their IM list and use that to keep tabs of when you're online.  They can send you email containing trojans and backdoors so that they can gain access to your computer and see what you are doing on and offline.
5. Harassment.  Anyone can use your email to sign you up for anything that generates a lot of email.  They can flood your email account and force you to get a new one.

How to hide your email address in this group:

1. Go to the What's New page
2. Click on My E-Mail Settings from the list on the right-hand side of the page.  You will be directed to a new page
3. Uncheck the box that says Allow other group members to see my e-mail address
4. Scroll to the bottom of the page and click Save Changes.