In order to add an open role to your project on the website, go to your project page and click 'Edit project'. You'll notice that lots of fields have been highlighted. If your project is currently closed to collaborators, you'll need to toggle that to Open so other users can request to collaborate.
Once the project is open to collaborators, you'll see the 'Add role +' button; simply click this and add roles by searching for key terms in the search box. You can add up to 6 open roles at a time.
Navigate to your project page and tap on 'Edit project'. Tap on the 'Collaboration' tab and go to the 'Looking for' section. Here you'll be able to add up to 6 open roles by searching key terms or scrolling through the list.