Just a word of warning, if you are wanting to have just any random join your world... be prepared for possible and likely griefing. There is no way to properly protect your home and belongings. There is a craftable item called a ward, which is supposed to "lock" your doors and chests from other players but, it merely just slows them down rather than keep them out.

With the ability to use console commands, a random could (as just one example) summon in mobs of ANY level... Ive seen posts of people encountering such griefing, discovering that someone had summoned in hundreds and hundreds of super high level trolls, which crashed the server anytime someone got near that area, and therefore almost impossible to "fix" that problem.

Basically what I am saying is, there are not good enough admin abilities to keep control over your world and prevent catastrophic mayhem from happening, and sadly, there are far too many people who just have no common courtesy, and would jump at the chance to ruin your world.