How to give another user full access to the user's www-data folder?

Question

ubuntu

I immediately apologize that the question, of course, is a lamer one, but I have never administered Linux, but on occasion I urgently had to.
There is a server, remote root access via sftp and ssh is disabled on it. On the server, nginx is running from the www-data user. Accordingly, the folder with the site also belongs to www-data.
Tell the teapot how to correctly grant access rights to this folder via sftp to another user (for example, user) for reading / writing and not disrupt the work of scripts and other things?
Understood that it is necessary to dig towards groups, but not up to the end.
Thank you all in advance!

Answer 1

Include the user user in the group www-data
usermod -aG www-data user

Answer 2

I think it will be correct through ACL
or add two to one group and do chgrp