When you first set up a Wagn, going to its front page will invite you to give an account name, an email address, and a password. This account will automatically be given the Administrator role. Once you are signed in, you can click on "Invite a Friend" in the upper right corner to give accounts to more people. See accounts to learn more about how they work, and configure other ways of people getting accounts.
Account options and most other general configuration is done on your Wagn's Config card — there are links to it from your sidebar and front page. From there you can customize your title and home card, logo and colors, the sidebar, and more. Many sections link back to documentation here for anything complex enough to need that.
You'll want to think out permissions as early as possible; while you can change defaults for new cards later easily, changing any existing cards' permissions has to be done by hand. Wagn's default setup is that anyone can view your wiki, but to edit they have to have an account and be logged in. You can alternately configure Wagn to be open-edit (anyone can edit most cards), or completely private (only people with accounts can even see anything).
The *default setting determines default permissions for all cards, except for those formatted by a *structure setting. Note that you can always change a card's permissions individually, even if it's controlled by a *structure setting.
If you're concerned about spam then you'll probably want to turn on captcha.
Developing your Wagn
Also see administration.