A search for scrape facebook group content shows there are a bunch of tools & services claiming to do this, for a price. Then you’d need a Discourse import script written. (And I don’t quite know how untitled group posts would import into Discourse.)
Short of figuring out how to import everything, is there any benefit in just trying to “simplify” account creation..? 
Registering for a forum doesn’t seem any harder than filling out a Google form, and if there’s extra special info you want to collect, it could be included as user fields on the signup.
If you collected user info via a Google form piped into a spreadsheet, and exported them to a CSV file, and imported them into Discourse, creating staged accounts for everyone… they’d all still have to make the effort to confirm their account and set a password anyway.