The last revision to this message I see is “You have been added to group $4 that have pages assigned to them”.
This message needs a documentation update, a GENDER magic word for “You have been” (so the name of the current user account must be passed as a parameter) and possibly a PLURAL magic word for $4 if it still can be a comma-separated list of groups.
EDIT: I noticed similar errors in other messages: MediaWiki:Notification-bs-pageassignments-user-group-remove-body, Notification-bs-pageassignments-user-group-remove-subject, MediaWiki:Notification-bs-pageassignments-user-group-add-body, MediaWiki:Notification-bs-pageassignments-user-group-add-subject. All of these are marked as outdated in the translation tool.
The group manager's username (the person who is adding or removing the addressee from a group) is omitted and probably isn't necessary in any language, but the addressee (the currently logged user who's reading the notification) is mandatory.
Thank you, and I hope my explanation isn't confusing.