Using @EVAL with a regular multi-select snippet might work for you (you'd need to install the Rowboat extra):
TV:
@EVAL return '-- choose a user --||' . $modx->runSnippet('Rowboat',array('table'=>'modx_users','tpl'=>'userOption','outputSeparator'=>'||'));
userOption chunk:
Making that work to restrict access to the resources, though, would be much trickier.
Another approach would be to skip the TV, create user groups and resources groups, and let the
MODX security permission system handle everything for you.