...
Summarize the group’s work
Update the community on group's accomplishments. See Tools for Publicizing Working Groups, Recruiting New Members and Onboarding.
Express appreciation to group members for their contribution
Email to the appropriate list(s) of the group’s closing
Indicate on the wiki and / or website the group's completed status
Remove calendar item from people's calendars (if applicable)
Update the Salesforce Committee record for each member, with the term end date
Close Sympa email list after a period of time (optional, there may a reason to keep the list around in case follow-up is needed)
...