Right now, you can only sort your Table Frame based on the fields you are showing in your layout. It would be nice to be able to sort the data "behind the scenes" on any field, not just the ones that you are displaying in the layout.
Using version 2.7, I just found that a sort cannot be done on a field of a joined table in the table view for a table that is joined based on a relationship class. So, based on your post the joined table has to be added to the project independently in order for the sort capability to work?
As a work around you can have the sort field turned on in the contents pane, open the Format Table Frame Field pane (right click the sort field -> properties), under Appearance make sure that the Auto Width box is unchecked and type 0.0001in into the box. The column is so thin in the table that it doesn't seem to appear in your layout.
Thank you for the comment @JordanSeider I took a look and it doesn't appear that you added your kudos to the idea. While comments are important, kudos (the number of votes) that an idea receives is one of the primary pieces of data that we use to determine traction and trends as we monitor idea activity over time.
