multiple signers editing a single text field

How would I get multiple signers to be able to edit a single text box? I have a form that I need 5 people to sign. On that form are some text fields that any, or all, of the users need to be able to edit. I thought the "All" signers option should allow me to put in a text field that could be used by all of the signers but the option is greyed out and not available to me. 

Does anyone know of a different way to do this or am I doing something wrong?