If you're the first person from your instance to subscribe to a community, you have to follow these steps:
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Open up the "Communities" page in a browser
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Search for the community with [email protected]
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Switch all search settings to "All" (even "Communities")
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Wait 5-10 seconds for your instance to pull in the top few posts
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Click through to the newly pulled in community and subscribe
Now that community is pulled in to your instance, and will be available for both yourself and all other users on your instance