We have usergroup-a and usergroup-b.
Users from usergroup-a can only see events in category a
In event booking we have 2 categories: a and b.
When a event is created in category a, users from usergroup-a have to be informed that there is a new event.
For usergroup-b the same must happen, but for category b.
As mentioned, you will need to develop a plugin if you want to have this feature. If you don't know how to develop plugin, maybe you can consider hiring us (or someone else) to develop this plugin for you
The price for this development would be 100$. It will take 2 to 3 days to get it completed