Groups are also not an asset that you own and have control over and it's VERY difficult to try and get people in a group to move the discussion over to a new forum later - I've never seen anyone do that successfully, though I've watched several try it.
So I first wanted to touch on this - you're right you don't own them. However you can require an email address entered to join (make it a private group) which then translates to an asset you DO own (an email list). This can then be used to promote the forum or whatever else. As you said, forums do have a unique ability to catalogue and archive information in a way that's much more difficult than groups but groups are easier to gain members. By building the email list, you could then weekly, send email updates about exciting/hot/interesting topics/posts/threads from the forum to group members on your email list (very similar to this forums weekly hot topic update) that would help drive some of the group traffic to convert to users on the forum. It's not a perfect system but it may help in initial growth.
Another tactic might be writing guest content that then points back to your forum - Join the Conversation over at XYZ.
Other, albeit more intensive ways, could be a podcast/and or YouTube channel centered around the niche to help drive top level awareness that points people back to the forum.
Quick Edit Here: Also, doing interviews on OTHER podcasts/YT channels can be a good way to gain awareness and then point people back to your forum.
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