From LedHed's Wiki
Creating Groups
groupadd
groupadd -g 2001 groupname
-g = GID
Creating Users
useradd
useradd -u 2001 -s /bin/false -d /bin/null -c "Users Real Name" -g group username
-u = UID
-s = Shell
-d = Home Directory
-c = Comment (User's Real Name Typically)
-g = Default Group