User groups are sets of user accounts that have access to the same groupware items (folders, calendars, etc.) but can have set different security privileges. One user account can be a member of none, one or more user groups.
Group accounts contain lists of member accounts. These members can be users, mailing lists or even other groups. Groups can be given access to any shared folders defined on the system.
Individual users can share their calendar and contact information with groups, as well as other individual users.
Emails can be sent to group accounts, they will be either routed to all group members or stored in a shared folder accessible to group members.
This functionality allows, for example, creating a group for a corporate department and defining a common data store (shared folder), common address book, common calendar, etc. Any changes in these common stores are immediately available to all members of that group.
There is the Message tab within group accounts. You can define your header/footer, subject and reply-to/from headers just like in a mailing list. This gives you the power to define group accounts in more details.
See the Domains and Accounts Management Groups chapter.


NOTE: The "closer" setting of access rights has the higher priority. I.e. access rights set for a user will override rights set for a domain. Example: For domain.com, access rights to some folder are set to Read, for the specific user (e.g. john.doe@domain.com), access rights are set to None. This means that John Doe has denied access to this folder regardless of the fact he is a local user of this domain.
Example:
You want to create a shared email folder, called "Support", to keep the appropriate employees informed about your customers response, group contacts, calendars, etc. You want to have one group of subscribers for this folder. Two groups (Developers and Quality Engineers) and three users should belong to this "feedback" group. Developers and QEs should have read, write, modify and delete access rights, three other people should be just informed about customers responses.

The Support folder will appear in email folder structures of all group members.