Simplest and Sure manner adding multiple Groups and Users, CentOS 7, macOS
The easiest way to add a group account on macOS is to use System Preferences/Accounts, click the + button and select the type of account to Group



However, if you are looking for a simple way to add a group account in command line then other than the dscl option there is also the Directory Service command line tools. Type following in Terminal:

dseditgroup -o create -u myadminuser -P myadminpassword -r "Some Group" somegroup


That will create the group account in the /Local/Default domain (same as the above in GUI).

You can add an existing user to the newly added group with:

dseditgroup -o edit -u myadminuser -P myadminpassword -t user -a someuser somegroup


and check membership in the group with:

dseditgroup -o checkmember -m someuser somegroup


It will display either:

yes someuser is a member of somegroup


or

no someuser is NOT a member of somegroup