Here is something dead easy you can try and it will probably work.
You will (temporarily) lose all user accounts and history, so it best to back up and run off a clean BugNET database.
Open your web.config for BugNET in IIS (Under ASP.NET Tab, Edit Configuration button)
You are now in "ASP.NET Configurations Settings" Dialog (editing your web.config file)
Step 1:
Create a new connection string pointing to your Community Server SQL database. (This is where your users for Community Server is stored)
Step 2:
Under the Authentication tab, Choose to manage providers. In this case our default provider is "AspNetMembershipProvider"
Click on "Manage Providers"
Step 3:
Edit the connection string property of AspNetMembershipProvider to the connection string you created in Step 1.
Edit the Application Name to be the same as Community Server.
Step 4:
Ok. Open web.config for both Community Server and BugNET.
Copy over your MachineKey setting in Community Server's web.config file and overwrite the machine key it in the BugNET web.config. This will make the encryption keys for Community Server and BugNET the same.
Everything should be fine. BugNET will get it's authentication from Community Server user base.