Guild Friend List
 

An addon that allows you to add guildies to a friendlist that will tell you when they login, as long as they are in the guild. That way you don't have to tie up friendlist spots with guildies. Idea being, the api should be able to figure out when guildies log in and out.

I think you can solve your request much easier.
There are in-game API events when guild member changes his online status (EVENT_GUILD_MEMBER_PLAYER_STATUS_CHANGED), so you can add event listener to this event in your addon and print into chat custom notification.

No need to misuse friend-list system.

GuildNotificator ?
But you cannot choose which player will be shown. It's for the whole guild.

This might help out...
 

Tried out some new skills for this addon!! Let me know if you like it and if it's helpful to you, and I'll add more slots for your selected Guildies!!
Not sure how to post a link to it here in the forums, yet, but it's called ESOPager!!

Lets you know when your selected Guildie's online status changes and when they change zones! Plus, a bonus button by their name to jump you straight to the closest wayshrine to them!!

Link to the addon of Minceraft:
ESOPager

Once again, thanks Baertram! Now i know how to do that as well!! And MailBuddy seems to be working awesomely!!