The command groups prints on standard output the groups to which you or the optionally specified user belong. Each user belongs to a group specified in /etc/passwd and possibly to other groups as specified in /etc/group. Note that /etc/passwd specifies the numerical ID (gid) of the group. The groups command converts gid to the group name in the output.
The output takes the following form:
example% groups tester01 tester02 tester01 : staff tester02 : staff example%
See attributes(5) for descriptions of the following attributes: