Enabling Plugins for Multisite users.

I have set up my first Multisite - and appears to be working great. I have installed the Dashboard under the Network Admin Dashboard - and installed a few plugins to test.

I have also tested the creation of a new user/site from the backend and frontend - this is all good too.

However, something I can't work out.

My Home Page/Main Site - "the hub" that describes the network and offers instructions and sign-up etc. Will most likely have special plugins not available to the other members of the network (Like the Q&A Plugin). How do I add a plugin that ONLY my Home PageMain Site can see/use?