Originally posted by Suzaku:

Servers are password protected because some people suck. If you invite someone to your world, you need to erect wards everywhere to protect your structures, or else another player can just go up and delete them, or take stuff from your chests and then never return.

If you want groups, then I recommend using the LFG section here to find a fresh world to join other people and build together. The system generally works best when playing with friends, as you're less likely to get a troll to ruin everyone's good time.

The mentality with running a server is this: my server, my way.