I am not an expert, but my understanding is that you have to use the search function (Top right of the page)

1. Copy the URL of the community you created
2. Paste it into the search box
3. Wait (It can take a little bit, it will say “No results.” at first)
4. Once it shows up, you can click in and subscribe

By default all instances don’t send all activity to all other instances (that would get crazy as activity scaled up). When someone on instance A (reddthat for example) subscribes to a community on instance B (dbzer0 for example), then the instances will start sharing the activity between each other