As well as providing e-mail relaying and archiving facilities for the many lists it hosts, the Yahoo! Groups service provides additional functions on the web site, such as voting and calendar systems and file uploading. The basic mailing list functionality is available to any e-mail address, but a Yahoo! ID is required for access to other features.
The Yahoo!Group has 1 or more moderators. Moderators serve at the pleasure of the group owner, and the owner decides what privileges a moderator should get. Note that the owner is also a moderator. There are also members and anonymous visitors.
We generally think of the owner, moderators, members and visitors as individuals or people. Actually in the definitions above, the owner, moderators and visitors mean individual email addresses. So the same person with multiple addresses is considered different people for the Yahoo!Group.
When a group is first started, the Group owner defines the purpose and sets up policy subject to the Terms of Services of Yahoo!Groups. In order to do that, the Group owner has to provide an email address, and get a Yahoo!ID. The Yahoo!ID is like an login to Yahoo for using Yahoo Services.