Still no "Last Online" System...?
You can use the leaderboards to determine when someone was last online. If you don’t want to go through them manually, you can try this.
Considering the ability to do this has been available for a long time now, they should have really implemented it in-game by now.
Anet recently gave an interview where they said they where not working on guild-management and it had no priority.
While there is a lot wrong with it and it needs a lot of attention. Then again, if they keep going on with the living story then I guess there is indeed no need for it anymore.
Anyway, on this thread on page 4 you can see a full list of options that should be available for guilds.
https://forum-en.gw2archive.eu/forum/game/suggestions/Guild-System-Improvements/first
It’s a collection of many topics about this subject.