NOTE–this post has been updated as the original solution was insufficient to bring the button back. In the author’s defense, he tried several things to fix it and misidentified the real solution. Thanks to commenter Matt for pointing out the error of our ways.
In CRM 2013 SP1, the Associate View subgrid of contacts no longer displays the “Add Existing” button. This is fine if you don’t use it, but what if you wish to link existing contacts to an account?
To make it show up, create a solution that just includes the Contact entity
Open the solution in the Ribbon Workbench.
On the contact subgrid, you will see two buttons that say “Add existing.” Right click on the first one and select “Customize Command.”
Right click on the command for MSCRM.AddExistingRecordFromSubGridStandard and select “Edit Display Rules.”
On the right pane of the display rules selector, select the rule called “MSCRM.NotOnAccountForm.” and click “Remove”
Publish your changes and reset your browser temporary files, and you should now see the “Add Existing” button on the contacts subgrid from an account. Note this will not display additional buttons on the inline subgrid, but if you pop the subgrid out using the grid button or access contacts from the “Contacts” link on the navigation menu, you should once again see the “Add Existing” button.