Welcome to addon development.
Please read the sticky threads first before posting
Also check the other forum about tutorials:
https://www.esoui.com/forums/forumdisplay.php?f=172
and addon help/search (
https://www.esoui.com/forums/forumdisplay.php?f=164) as they contain most asked questions and answers already as sticky threads.
You would know then what is possible, and what is not (your idea is somehow possible, as Votans Addon linked below shows, but there is no way to show floating markers above enemies/NPCs/Group members) then
New to addon development?
https://www.esoui.com/forums/showthread.php?t=9867
-> What addons can/cannot do
https://www.esoui.com/forums/showthread.php?t=9865
btw:
Your addon idea already exists:
Votans Group Pins
And I do not think this is an easy "starter addon", it's advanced. If you want to start with any simple addon buid yourself a "standard addon template" for your addons, to learn the difference between locla and global variables, muliple lua files in your addon, SavedVariabes with server dependent saving, settings menu with LibAddonMenu e.g.
If you have build your own template for your addons and have created your own first small addon to change any small thing that you disliked all the time in the game, then advanced with bigger addons like this.
This is only my advice, feel free to directly start with a big project if you like to ;-)