Procedure involving business partners

The start is an instruction send by us normally via e-mail. This triggers a process, where the procedure to follow is dependent on certain particulars of the case. So I have created a form where I want to start the Form just via a link to then upload this instruction. Once started, my business partner should be notified so that he or they know there is something to pick up. 

I want to test starting a Snap Form as a Standard User and as an External User. The SnapForm is assigned to a User group including my business partner and their User, as well as myself. All Users of this group should be able to reassign.