If you remain in the free tier, all your users are free, but you won’t have admin features. As soon as you buy just ONE admin user, then every user with emails in the same domain will need to be paid too.
If you buy paid accounts for like sitepoint.com emails, then anybody NOT on a sitepoint email can still be a free user and only have access to groups/projects they are invited to.
I’m quite familiar with Mango but one of the issues it won’t handle as well as Trello is with sub-groups. For example you have a group for the PHP Channel for example, but then you also need a sub-group for suggestions, available topics, in progress, ready for edit etc.
I was thinking about this the other day, browsing my own Mango account trying to figure out how to organize the workflow of article editing. Use files only? Use one Post per article with tags? Use Wiki?
I don’t think Mango (or most other intranets) can do quite the same thing as Trello does by having multiple card topics within a general group. But then Trello doesn’t have the user permissions and document versioning and social aspects like an intranet does.