Here’s a quick tip for anyone that may be having this issue that I noticed today. It can be frustrating when things don’t work as they’re supposed to… I do like it when the fix is something simple though!
I was trying to add a Contact to another (custom) record today and found that the “Add Existing Contact” button was unresponsive. The button was just not working. After a quick investigation, I noticed that the button was not working when the “Editable Grid” was shown in the Associated View.
Once I clicked on the “Show As” drop-down menu item and changed the value to “Read-only Grid” the “Add Existing Contact” button was clickable and working again. Once I was able to click on the “Add Existing Contact” button and add my contact, I was finally able to get back to work… and get on with my life.