http://www.artificialart.co.uk/acf-user-search-select.zip
https://github.com/artificialart/acf-user-search-select
Above setup as plugin. Tested on ACF v4 & WP 3.6 only
Works with repeater field
Will submit to http://wordpress.org/plugins/add/ when we get round to readme and screenshots
To use just start typing into user search field- results should appear below, then click on result to select.
Hope it works for you. Please let us know.