I assume you mean change the max amount of players that can join a server? Well this is simple to do.
Now your done!
Note that if you go into 'Build mode' then you will have to click the Tools button at the top of the screen to find the Explorer panel and etc.
Also if you can't find the panels even after that then go to View and select Explorer and Properties.
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The Roblox admins are people who actually work for Roblox as they're job. if you want to become a Roblox admin you would have to work for Roblox
You exit the server you are currently in and go to a different server (that's not already full) and click the join button.
You either create a roblox server yourself, or you gain the trust of the owner of another server and ask him if you can be Admin.
Roblox will do it, you can't chose it.
Some (but not all) games on Roblox include scripts which allow you to vote to kick players out of a server. This is usually done by chatting "kick/user name here". If enough people vote to kick a player from the server, the player is removed.