users – working together – Ask Ubuntu

I want to have different users including www-data to work together under a specific path. including updating each others files.

I made a new group webimage

addgroup webimage

and

adduser user1 webimage
adduser user2 webimage
adduser www-data webimage

I changed the permissions of the imagedir

chown -R www-data:webimage image/

and user1 can write but the file has permisions

-rw-rw-r-- 1 user1       user1

and no one can update the file but this user. How can I get this to work in a secure manner. Outside of this directory of course each users file is private as usual.