Pushing other people's patches to gerrit

Hi,

I've heard already from 2 people that it is not possible for them to
push other people's patches to gerrit; they get an error like:

remote: ERROR:  In commit XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
remote: ERROR:  author email address YYY@YYY.ORG
remote: ERROR:  does not match your user account.

I wonder - what setting needs to be set for them so that they can push
other people patches?  Or do they need to get the full push rights?

And in general - what's the reason for such a restriction?  Why somebody
cannot eg. backport somebody else's patch to a stable branch, even if
they don't have full commit access yet (?) - the patch will be reviewed
anyway...

Thank you,
Kendy