Hi Adam, the first thing you should do is create a site on a hosted server and play, like you did when you were a kid. Just hit every button and see what it does. All the things you ask are built into the program because thousands of people like yourself want these features.
As Markie said, you can set the site up to allow anyone to sign up, or invitees who you can email from the site itself. See Admin/Users/Browse Users/Invite New Members. While there Look at Suspended and Unverified, they also have important tasks.
Who you allow to join is located in Admin/Privacy & Permissions/Global Privavy
From the outset, two types of users can access the site (Free and Guest). Both are set up in Admin/Users/User Rolls/Permissions (This last link is at the bottom of the page). Guests have very limited access, but you can make it so they can't even get in. Then you can restrict what free members see from a massive list. Be warned that his list grows as you add more plugins so you must come back and set the permissions for that plugin. You can then create any number of different user roles such as silver, gold, admin etc and give them all kinds of different access privileges. You can have it that some can see photos and some can't.
It's all there and just clicking on each Admin Icon produces menus which often have multiple choices.
So, on your new account create two or three roles, add two or three users and play. You can change a user's role back and forth by going to the Users profile and clicking the Role button near his Avatar. Only Admin(s) can see this button.
As the creator of the site you have full privileges. DO NOT assume that because you can see something, everyone else can see it too. That's not the case and it won't be apparent until you log off and sign in as a user with lessor privileges.
HINT: If you have two or three browsers, you can log on as two or three users (one on each browser) and send messages to each other or compare what each user can see.
Oxwall can be set up a million ways and your most demanding needs can possibly be met.
Maintenance mode closes the entire site down except for you. It won't do what you want.
New User John