A professional association can be a tremendously powerful entity, or it can be a waste of the member's money.  The judgement of this value is in the eye of the user.  Unfortunately, so is much of the responsibility of engagement with other members, making use of available resources, and clearly seeing a link between membership and personal benefits, as well as benefits to the user's organization.
It is precisely this reason that websites for professional associations have such potential power and value for a professional organization. 
Often times associations, or the vendors working with them on a website will throw up their hands when it comes to letting the members interact with the website.  Building a social network, or web 2.0 features  is complex from a strategic point of view, but with the available tools, drupal, and other CMS systems, technically, it is not that difficult to implement, once you have a clear idea of the "why" behind the 2.0 features.
Some common issues that arise when it comes to user features.
Problem 1 -  My users have all kinds of different emails and passwords, they even have a password for their national account (that we don't have access to) so I don't want to confuse them with too many differnt accounts.  We're transferring them from another system and don't want to make it too difficult for them.
Solution - In a case where you're dealing with a bunch of information for users that already exists, it is fairly easy to import these users into a new system.  Generally the user's email is one key piece of information that identifies the user as unique.  Retain this informaiton and trust that sending them an email to this address will be successful. 
Sometimes user's get grumpy if they have to have new account information, but if the value of your new site is such that it provides more benefit than the inconvenience of the new account, you shouldn't have much problem asking users to do this.
Even more simple is to communicate to the user with the existing account information and allow your system to accept OpenID - if your user has a gmail, yahoo, aol, hotmail, or other common, free email, chances are its supported by OpenID.  So, send them an email telling them to go to the login section (if you don't want to send a password, have them click on a retrieve password link), once they log in, have them register their account with OpenID and they have a common email password combination for accessing the site.
First things first for your users.  Get them to verify their current user information and incent them to fill in any new information that you want to have as part of new data structures, new web features, and new organizational needs for your data.
Think first about the user directory.  This is a hugely popular aspect of professional association sites.  Users joing association sites to benefit themselves first, be it through improved knowledge and opportunities for their existing employer, or in order to search out new opportunities through networking, the member directory is a heavily trafficed part of any association website.
Are there data associations and options for organizing data that were not available to you and your users with an older technology?  Tools like Drupal allow you to create automated collections of data based on common aspects of data, and also taxonomy. 
For instance, if you are using Drupal, and you have imported your list of users, you can automatically create links of users that have the same title, who work for the same company, have the same name, the same zip code, etc.  Professional associations, and humans in genenral, look for things they share with each other as an entry point to socialization.
The user profile system in Drupal allows this to be done pretty much instantly.  So there's no reason not to import this information.  Next is to get your users to correct and update thier information.  Drupal allows user's to manage pretty much every piece of information contained in their profile.  You can also allow the user to set permissions of what information other users and non-users can acccess.
Even if you don't want to import information, just invite your users to register at the site, and then set the user settings to "administer must approve all applications".  Get a registration that isn't a valid user?  Reject the membership application or send them information on how to join and mark the account "inactive" until you have further data.
One goal that needs to be communicated to members is that by loggin in they get access to informaiton and site features that non-members don't get.  The value of this is twofold, one, it helps the user justify their membership dollars, and two, it encourages new users to become members in order to access the information on the site.
Some simple examples of this:

• Offer article and report abstracts to non-members and full articles to members
• Offer only partial contact information or access to contact mechanisms for non-members versus members
• Allow members to associate with the site taxonomy so they are directly and indirectly associated with larger content groups, indexes etc.
• Ability to post comments, ask questions.
• Ability to not only answer polls but to see consolidalted results of polls.
• Access to archival data

With most modern content management systems like Drupal, it is fairly easy to use access permissions to differentiate the user experience both with limiting access to content or to community features only to qualified members.
You only get one chance to wow your users with a new website.  Don't launch with the same old same old, a site of static information about all the benefits of membership and a bunch of publically available information.  With the systems available today, there's no excuse not to bring your members in early to hlep you help them build the professional association website of their dreams.