Simply go here. https://groups.yahoo.com/neo/groups/all/list
It will give you a list of all the groups you are a member of. If some are missing do a search for them by name and contact an owner or moderator to see if you are still a member of the group.
Yahoo Groups are not part of Yahoo Mail. Help for Yahoo Groups: https://help.yahoo.com/kb/groups
Help for Yahoo Groups https://help.yahoo.com/kb/groups