You cannot force individuals to join your Facebook group! though you can suggest it!
Asking them to join on the Thank you page is probably the best option you got, or in case you want to make sure you’re getting near perfect joining rates, just add a link in your thank you page that asks them to Join your Facebook group before proceeding any further. Once the hit the link open your Facebook group link in a new tab… And show them a popup on your Thank You page confirming whether they’ve joined or not. if they confirm you got nothing but to trust them