Secure Login
The ability to provide your users with Secure Login facilities is key to building a strong, robust user base. This would be advantageous for providing more secure information or details to group/site members about pricing, or property if you are an Immobilier for instance.
There is some very insecure functionality offered in the market so it is important to make sure that your level of security matches the information you hold or wish to disseminate.
Membership
Allied with secure login facilities are Membership Systems - these can be as simple or as complex as you like, but invariably they include some elements of a Forum (something I avoid generally), but if you really want one; who am I to complain.