To limit two groups, one to each of two events (I am going to use Group A  and Event A. NOTE: as a pre-requisite, you must assign the event a unique fund) you can:
1. Log in to RE as a supervisor user
2. Go to Administration > Security
3. Click New Group, call it "Group A" and make sure that they have all of the necessary rights to access the plugin and process your transactions
4. Add users that you only want to access Event A to this new group
5. Go to the NetCommunity plugin, and click Options
6. Click Transaction assignments
7. For "Transaction Type to manage" select "Event Registrations" and check the box "Enable Assignments"
8. Select "Group A" in the "Security Group" box
9. Select "Fund" in the "Filter Type" box, and then select the unique fund for Event A in the "Criteria" box
10. Click "Add" and ensure that in the table below, the box for "Enabled" is checked and click OK