... need to be able to select USERS in many categories ... = LINK OWNERS

... do I need to add a field to the user table ... = NO






That should be easy to do ... NOT!

Go to your Links SQL Admin area

Find: "Email" > "Selected Links" > and at the bottom select categories to which you wish to send emails

It's not easy because it doesn't work. At least with version 2.1.1
Also, you can't save email templates ...