You should set up a Twitter account for your game.
You’re allowed to promote your Discord group on your Twitter. You’re not allowed to link to any tweets mentioning Discord, though, and you’re not allowed to mention, reference, or suggest Discord on the Roblox website at all. Players who check out your Twitter may also check out your Discord if you promote your Discord group on Twitter.
You can check out the already-mentioned Reminder Regarding Permissible Links thread for more information about exactly how the rules apply to this scenario.