A super-simple way to email groups. Something where someone can easily subscribe and unsubscribe. Something that doesn't require signing up for an account, but only add an email address.
Concept:
Go to www.saturdaymorningrunninggroup.emailgroups.com
The webpage will have a box to easily add your email address.
To email the group, you send an email to saturdaymorningrunninggroup@emailgroups.com and it is broadcast to everyone on the list.