I have realized in my short experience on BB (since season 20) that contributions to forums are what keep new users involved. When a user is engaged by other users, it makes them feel less lonely in a game that takes a lot of patience and learning. Alot of countries have started a "Mentorship" program where new players can connect with seasoned veterans for advice and guidance, but I believe the most effective way to engage new users is for the individual league's forums to be active. If you see a new user pop up in your league, shoot them a message and offer advice and conversation about their team, the league, your past blunders, etc. Unfortunately most new users start off in bot filled lower leagues, where there might not be that much forum activity.
That's just my two cents.