Email: no. Clients can forward documents to their Briefcase email address without an account.
WhatsApp: yes. WhatsApp uploads are matched to the phone number on the client's Briefcase profile, so the client needs an account with their phone number added. When you invite a client, the invitation includes the set-up steps, and WhatsApp is set up during sign-up, so there's nothing to configure on your side.
Learn more: 📄 How to add users to Briefcase: invite team members and clients · 📄 WhatsApp upload · 📄 Email upload