Hey all,
Is there any simple way to allow some urls for guest posting? I want to create a safe environment so I'd like to have some control over the urls posted by guests and allow some major sites and block the rest. Thank you.

It depends on what kind of URLs you'd like to allow. Do you mean hyperlinked URLs? By default, miniBB wouldn't allow to post hyperlinked URLs until the certain amount of posts is riched. This is definite, and only a customized code would change this approach.

In other words, please explain how do you see "URLs for guest posting" in action.

Hyperlinked urls for sure. I'd like my guests can post youtube urls but not blablablabla.xyz type of urls.

Currently, we don't have such add-on or feature. If you have just a few of allowed domains/URLs, then there could be an easy way to program an addition to bb_codes.php which would allow to "hyperlink" such URLs disregarding the $userUrlsAllowed rule. Also, there is a way to add kind of special BB-code button(s) for such a few domains, introducing just a new BB codes which would work for everyone incl. guests.

However if you would like to have a rich library of such URLs, there would be more complicated code/add-on which we don't have at the moment. But I could think it of and you could be the proud sponsor of it in terms of our regular fees.