Give read/write permission of group files to a user

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.

for dir in /home/user1 /home/user2 /home/user3 ; do setfacl -R -m u:user1:rwx $dir ; done

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.