With this datapack, you can set custom join messages whenever a "special"/"VIP" or normal ("generic") player joins. Just so you know, this datapack likely won't announce you the first time that you enter a server. If you have questions or suggestions, please feel free to leave a comment. Here are the README file's contents:
/tag <Player Name> add joinmessages.special/tag <Player Name> remove joinmessages.specialSpecial players: special.mcfunctionGeneric players: generic.mcfunction/reload