same problem here, added co-authors, was able to add them on second publication from the selection, but not on third, now also not manually as the e-mail is recorded and it does not let me to add them again..

Hi Joris, thanks. It did not work on Chrome Version 35.0.1916.153 m. So I tried the latest Opera and it worked there..
BTW, not sure what the m in the Chrome version means. But it is not mobile it is on Win 8/8.1