Problem: There are many addons that have alt-name features, yet many require different syntax. It can be difficult or impossible to find a common syntax that supports all common addons that guild-mates might be running (I.e. Chatter, Prat, GuildGreet, Alts, ElvUI...)

LibAlts is a common library that provides a database of alts for other addons to use. In GuildGreet's case, you would simply need to read from it. It has a well documented API.

I suggest checking for that library, and if present, import (or read dynamically on event if possible) from the library database. With LibAlts as a back-end, you'd be compatible with the three most popular chat interface addons and alts addon.

https://www.wowace.com/projects/libalts-1-0
https://www.wowace.com/projects/libalts-1-0/pages/api/lib-alts-1-0
https://mods.curse.com/addons/wow/libalts-1-0

Other addons that use LibAlts:
https://www.wowace.com/projects/libalts-1-0/relations/dependents

Thanks for the tip. I don't know, that such lib exist. i will take a look inside after i move to mynew home :)