If they have the ability to create a blog post on your WordPress install, or on your multisite install, they can use those as long as they are using the Graphene theme.
I’m not sure about using them in a forum. Which forum plugin are you using?
Of course, you can always try this in a test post in the forum and see if it shows up. I don’t think you would need to enable any sort of permissions or anything. The Graphene code is unique enough that it shouldn’t be overridden.
Moved to Support.