Directing a user to an appropriate log in form

Hypothetically, a business has a suite of 10 products, each with separate domains. This business has separate 4 user groups who achieve different tasks on each product. Some products are made for only one user group, other products are made for all. Each product must have a separate login/registration form.

In a global sense, what is the quickest way for a user to be directed to their appropriate product and either register (as a new user) or log in (as a registered user)?