modify group for one command

I want to execute one command and I want that it is executed with a specific groupname, that I'm belonging to, but which is not my default group. So all files created will belong to that group. Is there a command for that? There is chgrp to fix it afterwards. Then I found newgrp (https://unix.stackexchange.com/questions/23093/changing-my-default-primary-gid-for-every-login-session) , but that creates a subshell. Thus a oneliner newgroup && cmd does not work. And the shell needs to be closed later. Both approaches looks error prone.