The help article link located at the “What restrictions exist on who can join a private server?” section seems to be no longer valid
The link “https://en.help.roblox.com/hc/en-us/articles/360031751471” seems to redirect to the roblox support start page
Page URL: https://en.help.roblox.com/hc/en-us/articles/205345050-Private-VIP-Servers-FAQ