I would consider advertising my Roblox groups on Discord server advertising sites, or perhaps the Roblox advertisement system. If I’m going for the cheap option, though, it’s best to advertise on Discord server finding sites.
Discord advertisements are a great way. But my favorite one would differ based on what I’m doing. If I were to be creating a cafe/hotel/airline group. Even though they may seem old and not worth it I would use Roblox banner advertisements as the amount of new users who click on them are relatively high. To advertise for a game I would probably send it through discord and ask some of my friends to circulate it within their groups. It all depends on what you’re doing.