Editing a wiki page
The easiest way to edit the wiki is to use the top bar, which will open up when you click "edit" to the right of the text box. This will apply a wiki mark-up language to your text- just type between the characters that appear. Otherwise just type!
If you want to create a new group page, create an internal link, by clicking the the third (underscore Ab) button, or inserting the page reference between square brackets. The full syntax for a news page would be dih.wiki.otago.ac.nz/group_pages/news, but you can just type (between square brackets) group_pages/news. This page is there already, but feel free to add pages and edit them.
Adding external links is easy, just click on the internet button, and type the URL, followed by a space, then the description.
Detailed editing help can be found here mediawiki help page
What are you waiting for? You won't do any damage!