Add customer participant en masse is used when you are enabling customer users, and need to add them as participants to processes they are referenced in. 

Top menu >> Customers >> Contacts tab

From the customer page click on the Contacts tab.

Click More, Then Add customer participants. 

Select the process you want to push the customers user to. 

The page will update with the number of forms it will update, click Start to begin the push.

Once the process is completed, a confirmation of how many participants have been added will appear. Click Done to finish.