A simple plugin that invites players to your Discord server if they haven't done so yet
It works by detecting the gamemode of the player and if it's spectator mode, it's presumed they haven't joined yet and the message is displayed.
You can safely reload the config by reloading this plugin with PlugManX.
(I do not recommend using it for other plugins! Plugin reloaders like this can seriously mess up some plugins)
Bedrock Edition (Floodgate):
This plugin has optional support for the https://github.com/GeyserMC/Floodgate plugin!
Bedrock Edition doesn't have clickable links, so for Bedrock players the link is displayed in brackets.