How do I restrict people from seeing a file or a page?

Please see Permissions

How can I get at Blogs, Calendar, Forums, Pulse, Gallery or Members List? Why don't they show up in the main navigation?

All pages in an Igloo community can be edited and have their names changed if desired; This means that any of the above pages could be called something different. An administrator can also choose to hide any pages they deem to be unnecessary from the main navigation menu.

They can do this from the "Navigation" page within the "Presentation" section of Control Panel.This does not necessarily mean that they are always inaccessable, web pages can have links directly to these features as needed while still keeping them out of the main navigation menu.

How do I hide a folder structure? (to make a folder look more like a webpage)

To hide a the contents of a folder to make it appear more like a webpage click "edit" on the  Action button (at the top right). Scroll to the bottom of the page. There are three options at the bottom under "Folder Contents": Show, Full Access Only, and Hide. Select Hide and then save. Now when you view the page the folder structure at the bottom of the page is no longer there. This is a good way to create Webpages; simply use folders and subfolders as the webpages and then use hyperlinks to navigate between them. The folders allow the page designer to easily organize related pages.
Related Links: Folders, Web page editor

How can I change the invitation email?

How can I change the logo?

Can I hide "IGLOO powered"? "Top bar"?