I would make that through groups and security permissions for the categories.
So for example:
- 1st create a group for registered
- each category has set a security rule for everyone = can read (Hope thats right, I am using German locale)
- additionally each category has set security rule for group registered = create, answer, read
Correct me someone who has better knowledge than me 
Edit: When create the group, you have an option Automatic → Users who register with an email domain that exactly matches one in this list will be automatically added to this group:
I am not sure if it is possible to wildcard domains inside that option field? So that everyone who registers with doesn´t matter which mail adress is beeing automatically added to that group.