Lead Magnets for Forum Promotion

Have you ever developed a lead magnet just for forum promotion?

A lead magnet is a freebie, usually a digital product like an e-book, guide, app, or something like that. You offer it for free to users to join your forum. They get a link once they join and you get a new member.

Are any of you doing this to get more members at all?

I think the lead magnets work when they solve a real problem. Some random pdf just to collect emails won't work. There was a checklist I downloaded, it was simple but useful. That's what made me join a forum sometimes ago.