Having the default one row and entering a value into a field and then clicking the ⛔ icon next to three fields "Full Name", "Gerrit Username" and "Phabricator Username" removes these values again.
It's not obvious what that icon does (it means "no entry" in my culture) and it does not have a tooltip on mouse-over.

• Replace the icon with the OOUI clear icon at https://commons.wikimedia.org/wiki/OOUI_icons#clear
• Remove the icon from the first row, as it only functions as a duplication of the "trashcan" icon below when you only have one row.
• Only show the icon on non-first rows (because once a user has clicked the "add user" icon below, the ⛔ icon seems to function as a way to remove a non-first row again)