If you want to change the user owning this file or directory (folder), you will have to use the command chown. For instance, if you run
sudo chown username: myfolder
the user owning myfolder will be username. Then you can execute
sudo chmod u+w myfolder
to add the write permission to the username user.
But if you want to add this user to the group associated with "myfolder", you can run
sudo usermod -a -G groupname username
and then execute
sudo chmod g+w myfolder
to add the write permission to the group.