Step 1: from the command line; heroku sharing:add <client's email> <--app yourapp-production> (--app yourapp-production is only used if you have setup a staging heroku app and a production heroku app already)

Step 2: from the command line; heroku sharing:transfer <client's email> <--app yourapp-production> That will send the ownership of the app over. If they have not setup an heroku account, they should get an email to set one up.