Customer login design:

si_users
id | email | name | customer* | group# | domain^ | password *

* where customer is the customer id that the user can access and password is stored using md5

# si_users_groups
group_id | group_name
1        | users
2        | customers

^ si_users_domain
domain_id | domain_name

if group_id = 2 then show cut down version of customer_details page (where customer_id = customer)

• showing only
  • customer name
  • account stats (total, owing, paid)
  • and list of invoices but the actions reduced to PDF and Print View

refer: http://code.google.com/p/simpleinvoices/issues/detail?id=44&can=3&q#c3