Every wiki page has an associated talk page, which can be used for discussion and communicating with other users. Talk pages can be accessed by clicking the Actions > Discussion. Simply edit the page as normal to add your comment. A talk page is actually very similar to any other wiki page, but it is in the "Talk" namespace, to keep it separate from the articles in the "(Main)" namespace. As with any wiki page, you can edit it, link to it, and view the editing history.
User discussion pages can also be used to communicate with other users. Sometimes, next to a user's name you will see a "Talk" link. Click this link to edit that user's Talk page. Depending on their preferences the user will usually be notified either by email, or the next time they log in to the wiki.