Anyone can. If you see something that is wrong, or needs a extra bit of explanation, or is missing some technical detail, or could just do with rearranging, just dive in and change it. Be bold!
You don't even have to create an account to do the editing, although if you do, you'll be better able to keep track of what edits you've done.
If you want to create a whole new page, you will have to create an account though.
Can I create a page about my project or product?
Sure you can. If it will be of interest to other Propeller users, it's welcome here. And who better to write about it than you? You might already have some content that you've written for the manual or your website that you want to reproduce here. But think factual and informative rather than plugging and advertisement.
Style
Think of the Wiki as an encyclopedia of topics related to the Propeller. Try to keep it impersonal. If you use the personal pronoun "I", that might make sense at the time you are first creating a page, but over a persiod of time, a page will have text written by multiple people, and "I" won't make any sense any more.
Don't worry about being comprehensive. If you omit things and they're important, someone else can add them later.
Editing
Just hit the "Edit This Page" button at the top of the page you want to change. You start with a Visual Editor, but you can change to a Text Editor which accepts standard wikitext markup if you prefer. If ever you get strange things happening in the Visual Editor, it's often a good idea to switch to the Text Editor to see what's wrong.
Creating a new page
Click "New Page" in the list of actions at the top of the column on the left. You'll have to be logged in to see it. You then get to choose the page name. Try to make this brief but descriptive of the content. It appears in lists elsewhere in the system and needs to be obvious.
Formatting
Title
At the top of every new page, place a title. Often this is the same as the page name, but here you can be more verbose. For example the name of this page is "Editing the Wiki", but the title is "Editing the Propeller Wiki". Format this as Heading 1, either using the drop down menu or by placing = before and after the title.
Section Title
Then for each section of the page, give a section title. Format this as Heading 2 (use == before and after the title) and subsections as Heading 3 which is marked with ===
Code
For multiline snippets of code, create a code section with place [[code]] at the start and end. For the odd keyword that you want to display in-line, make the font monospaced bold, by putting {{** at the start and **}} at the end.