You cannot do this by script, and you cannot do this in general. You can't mess with Roblox's Server, which includes the maximum player count being 50.
Despite the number of games with 100 players in a server, they are not yet available to the public. Developers who have permission by ROBLOX themselves, or work with ROBLOX tend to have access to features like these. However, I am aware that ROBLOX is planning to release 100-player servers by the end of 2018.