Most People Arrive Through The Back Door
Most of your traffic will come via search to specific community discussions (often very old discussions).
This means most newcomers won’t see the carefully crafted messages on your homepage.
They won’t see the large registration button or go through the expertly designed journey to increase conversion.
If you want to convert more of your visitors, you need to find a way to target messages where they visit. This means alongside existing discussions (or content), links from within discussions, or (sigh) pop-up boxes on the most popular discussions.
This also means your community homepage is better suited to regular visitors than newcomers. Prioritize content and activities for regulars rather than newcomers to the group.