When you first install cygwin and run bash, it'll usually display this error message:

Your group is currently "mkpasswd".  This indicates that
the /etc/passwd (and possibly /etc/group) files should be rebuilt.
See the man pages for mkpasswd and mkgroup then, for example, run
mkpasswd -l [-d] > /etc/passwd
mkgroup  -l [-d] > /etc/group
Note that the -d switch is necessary for domain users.

If you happen to be in a corporate environment with a huge active directory, then the mkpasswd(1) command can take a very long time. A quicker way to fix this issue (provided only the current user uses cygwin on this machine, which is most likely) is to issue this command instead:

mkpasswd -l -c > /etc/passwd; mkgroup -l -d > /etc/group

The mkgroup(1) command will still take a few minutes to complete, but typically there are much fewer groups than users, so it's bearable.