Everybody already knows... But flip the coin and say they didn't. If asked to get involved, first ask yourself if you want everyone asking you all the questions. You need to be sure you're ready for that first. Start with letting family know because if they hear it from someone who heard it from someone they'll be planning your funeral.... Get to support groups if there is one, and if there isn't ask your Dr if you can start one.
I was involved with the SW Center in Phoenix long before I became positive because they do good work, I had friends who also volunteered, had positive friends, and dated a few positive guys while negative and wanted to educate myself. I took part in the AIDS walk, volunteered at pride booths, marched with SW center during parade all while negative... nothing super impressive or distinguishing, but I had zero concern for whether others knew my status or made assumptions.
Am I missing something? Why would working or volunteering with an organization that works with the HIV+ population mean that you are HIV+? Surely you don't think that only HIV+ people work or volunteer there. Besides they all have privacy guidelines and wouldn't tell so why why would you have to?