groups(1)
Name
groups − show group memberships
Syntax
groups [user]
Description
The groups command shows the groups to which you or the optionally specified user belong. Each user belongs to a group specified in the password file /etc/passwd and possibly to other groups as specified in the file /etc/group. If you do not own a file but you do belong to the group by which it is owned, you are granted group access to the file.
When a new file is created it is given the group specifications of the directory in which it resides.
Files
/etc/passwd
/etc/group