players

An incredibly simple library, exposes roblox's Players:GetNameFromUserIdAsync and Players:GetUserIdFromNameAsync, likely with some modification.

I am unsure whether these functions error or return nil in the event of failure.

players:GetUsername(userId) returns the username a player given their user ID. players:GetUserId(username) returns the user ID a player given their username.

You can see an example of it being used in the LifeSensor documentation.