I’m thinking of making a game similar to Murder Mystery and other games that require a group of players to play against each other. However how do these developers get players to play their game? If someone joins and there are no other players they will obviously leave because there is no one to play against. I’m not sure what category this belongs to.
Thanks for reading
Advertisements. You can advertise your game for robux under create tab
Advertising
Have you created your own game and wondered to know how players will play your game on a way to do? Then the advertisement is here for the people who understands how it works.
Advertising can be the only way for the option to bring your players by joining your game. If you create your own and advertise your game, your advertisements will be able to appear on the website. Once appeared, the players will see your advertisement that pops up and play your game, and you’ll understand how much visits on your game you’d get! The better it goes; the more visits you see are by the count of the players. It can also get popularity when good developers creates their game with fun gameplay and genres, etc. and advertise their game by the people who played their favorite games on Roblox.
I hope everything understands and helps for you! 
How much robux would you recommend I spend on advertising?
First of all, you need a game that is able to keep players active so they stay in your game longer. To do that, I would suggest you to add maybe:
By keeping your players active in-game, it will create an accumulation and will have more chances for you & for the game to get many people in many servers.
Second of all, you need to ADVERTISE your game so players get to join it.
How much robux would you recommend I spend on advertising?
I would say it depends on your capacity and how much you are willing to spend for your game.
In my opinion, a great advertising is 100k. But again, it depends on how much you can spend. 10k is good too
What you can do to get more sponsor funds is to get investors on your team who are ready to put money and help you advertise, in exchange of a pourcentage.
Hope it answers your question 
You can try spending robux by the bid of amount on your advertisement that’s recommended anywhere, anytime; but also you can run your own advertisements by the bid of robux! All ads can run in 24 hours. 
So is advertising better than sponsoring your game?
I would say NO, sponsoring is better than advertising due to the fact that many players use ad-blocker on the platform so won’t probably be able to see your ad(s).
I think I just chose the wrong word in my previous reply ahah
An important part of group/team based gameplay is related to core gameplay design.
Short rounds or drop-in-drop-out gameplay is a huge part of ensuring players don’t get bored waiting and participate. Encourage play-till-the-end with rewards for playing and greater rewards for winning.
Make your lobby fun! Having spectate options or small activities they can do in the lobby is a great boost to making sure people stay till the next round starts as well. Personally I’m a fan of having a way to earn points outside the main game, even if very slowly. If I have to wait a few minutes for a round to end I’d rather do something useful with it. Spectating can be incentivised with rewards for how long you spectated or the ability to spend points to somehow interact as well.
Obviously as other have said, advertising is an important part of building a player base fast but I personally enjoy the feeling of building a community without it. You lose out on a lot of profit potential but that’s never why I’ve been in it.
Aye!! I remember your Medieval Roleplay game. One of the first games I ever got into. The keys and social classes were interesting. I really liked how what rank you were determined where you could go.
I remember you shut it down few years ago, why was that? Did it break with updates?
Haha, long time since someone recognized me. I technically never shut it down, however a lot of things did break when Filtering Enabled came into play since I made it so long ago. I’m glad to hear you liked it, it holds a special place in my heart that’s for sure.
Community building was always my retention plan, and listening to my player base and adapting based off their wants and my vision for the game was ultimatly what made my place popular.