MediaWiki uses underscores and spaces for user names pretty much interchangeably. However, Special:NovaProject&action=addmember insists on usernames with spaces. When you copy & paste the username from a URL it has underscores and for rtl languages replacing it manually requires a lot of patience :-) (cf. https://wikitech.wikimedia.org/w/index.php?title=Special:NovaProject&action=addmember&projectname=tools&member=%D8%B9%D8%B1%D9%81%D8%A7%D9%86_%D8%A7%D8%B1%D8%B4%D8%AF).
So Special:NovaProject&action=addmember should transform any underscores to spaces before using the username.
Version: unspecified
Severity: enhancement