Do you want to search in all categories or in this category ?
You can do that from the user's profile and on the right hand side you can find the button for Unsubscribe/Subscribe to toggle between.
You can also subscribe users in bulk from your current Contacts tab by marking them and clicking on More. Then select Add Contacts To List and select to which list you want to add them. This will subscribe and add the contacts to the selected list.
