Cygwin users and groups

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.