The syntax would be
chown user:group file ...
This usually (=when you're not root
) only works as long as you are a member of the same group (since otherwise by chown'ing files you could exceed the other user's disk quota).
If you both are in the same group you could also allow write access with
chmod g+w file
As above this will allow all members of group
write access.
If you do not share a common group, all you can safely do is to allow read access to the file so the other user can make a copy of it and edit his/her copy.