hmmm 
I just successfully invited a test account to my own site into a new topic in a category they have permission to.
Step 1: Secure category and create topic
Step 2: Create and send invite
Step 3: Receive invite and setup account
Step 4: Arrive at secure topic
Perhaps I am misunderstanding what you are wanting to do?
Edit: If you want it to just be a member’s only category, then you can add trust_level_0 to the permissions. But you need to specify a group that also has permissions in the group field of the invite for this to work:
