You can enable/disable customers registration from Setup -> Settings -> Clients -> Allow clients to register.
Your customers will be able to register at your url base where Radius CRM is install eq. yourdomain.com/crm/
If customer registration is disabled you will be able to add only customers from the admin area.
To know how to add a new customer from the admin area, please visit this link: https://radius.radiuspro.co/knowledge-base/article/how-to-add-a-new-customer