When deploying an intake form (based on the matter object) using Force.com Sites, the user may get an error when submitting the form that an error was encountered when adding participants for the matter. 

Why am I receiving an error when I create a matter through the intake form?

The workaround is to disable the auto-participant lookup function on that lookup field so that participants will not be auto-created when the matter intake is submitted. For more details on auto-participant functionality, see AdvoLogix Setup: Participants.