If you have just created this interest category and check the box next to "Yes, make this a default interest for all registered users." It will put all existing and new users into this interest categories opt-in group as expected.

However, due to bug E-67136, if this interest group was created previously, and you check the box next to "Yes, make this a default interest for all registered users." It will not put all of your existing users into this interests opt-in group but they will be added to the interest. If the group is rebuilt during an email send, the constituents will be put in that interest opt-in group as they are still in the interest.