https://leaderboards.guildwars2.com
Login, select the achievements category and filter by your friends. Mouse over the achievement points to see when it was last updated (when they were last online). If you want to add more people, the list is updated when you logout of the game.
If you have a guild however, you simply need to invite them. Their pending invite will appear in the roster.
If one of your friends isn’t being shown in that achievement points list, what does that mean? They haven’t logged in for awhile?