If you type in
at the command line without any arguments, you will get a list of the groups that your login id (user name) is a member of. The first group in this list is the default group, which will be the group that new files and directories you are creating are associated with for access permission purposes.
With the list command:
you can see the permission settings, including the group, for the files and directories in the current directory.
You can change the default group with the sg command.
If you enter a user name as argument to groups it will list the groups that the specified user is a member of. For example,
will list bwillis' groups.