It's already there. Hover next to each guildie's name and it'll tell you how long ago they were last online, most recent at the top.
FCO GuildInfo e.g.
https://www.esoui.com/downloads/info1265-FCOGuildInfo.html
Oh sorry, I did not read that properly. I thought you meant "see online currently" but not in the guild roster.
My fault.
I think there is no "online state tracking" addon for guilds/friendslist so far.
MAybe within Shissus Guild Tools somewhere but not sure, you'd need to check that in the features list.
There should be an updated version for the current APIVersion. >The original one was broken afaik.