You can (optionally) link tasks and deals to contacts by email-address.
My CRM uses and extends the RainLab.User and RainLab.UserPlus plugins. This allows you to easily create a user portal for your customers.
My CRM extends the user list with tasks. You can see the number of open tasks per user and the date of the last contact.
My CRM extends the user with a sales pipeline stage and a tasks tab.