How can I use setfacl to allow a user to have read/write permission on the home directory of any user that belongs to a certain group?
I'm assuming that you want user1 to have access to the home directory of all users in the group1 group.
I don't think that you can do that with one magic command. The best you can do is add an ACL to each home directory that you want user1 to have access to.
You could put a script in place to run that every day. It could look for all members group1 and then execute the setfacl command on their home directory.
I think with more explanation of what you're looking for, I could provide a better answer. I am thinking proper group permissions and umask settings would take care of this much more easily than setfacl.