Finding out the subscribers to a categories/Boards/Topics

I think you can get everything you are looking for with API V2 subscriptions (khoros.com)

So for a list of subscribers to a specific topic:

SELECT subscriber FROM subscriptions WHERE target.type = 'thread' AND target.id = '[message id]'

Count would be:

SELECT count(*) FROM subscriptions WHERE target.type = 'thread' AND target.id = '[message id]'

For a category instead of a topic:

SELECT subscriber FROM subscriptions WHERE target.type = 'category' AND target.id = '[category id goes here]'

And a complete url with encoding would look like this:

https://[COMMUNITY DOMAIN]/api/2.0/search?q=SELECT%20subscriber%20FROM%20subscriptions%20WHERE%20target.type%20%3D%20%27thread%27%20AND%20target.id%20%3D%20%271387%27

Or if you are doing this with FreeMarker I think it would look like this where ?url at the end encodes the url:

<#assign subscribers = rest("2.0","/search?q=" + "SELECT subscriber FROM subscriptions WHERE target.type = 'thread' AND target.id = '1387'"?url) />