In ACP > Manage > Privileges, select the category you want to allow guests to post in, and give guests the "Create Topics" and "Reply to Topics" privileges.
If you post a link Github - to get the teaser:
How do you like the idea to do the same, only with reference to the topic?
If you're after a general preview of a link's content, you'd want to try embedly (I can't remember off the top of my head if there's an embedly plugin for nodebb)
Don't think there is. But the api could be used to create one.