If you are just trying to assign group permissions for a file or directory, you may be able to do that through your FTP tool. For example, WS_FTP on Windows allows you to set user, group, and other privileges through a GUI. However, I don't think it allows you to change the group. At least not in the free version I have. As Will indicates, you may need to use chgrp from a shell.
If you need to set permissions for a lot of files or directories, I would follow Will's recommendation. The shell is your friend, just a cryptic little friend who speaks what seems at first to be a bizarre language from an Eastern European country suffering from a severe vowel shortage. And with lots of inconsistent abrvtions.