You can change the default group for all files created in a particular
directory by setting the setgid flag on the directory (chmod g+s <dir>).
New files in the directory will then be created with the group of the
directory (set using chgrp <group> <dir>). This applies to any program
that creates files in the directory.
Note that this is automagically inherited for new subdirectories (as of
Linux 3.10). However, if subdirectories were already present, this change
won't be applied to them. Assuming that the subdirectories already have the correct
group, the setgid flag can be added to them with:
find . -type d -exec chmod g+s {} \; (suggested by Maciej Krawczyk in the comments)
If the setgid flag is not set, then the default group will be set to the current
group id of the creating process. Although this can be set using the newgrp
command, that creates a new shell that is difficult to use within a shell
script. If you want to execute a particular command (or set of commands)
with the changed group, use the command sg <group> <command>.
sg is not a POSIX standard command but is available on Linux.