I want to create an address list to add external contacts and give access to a couple of users to add and maintain the address list. This list should be available, while composing email address, users can select the contacts by clicking on "To" button. what is the best way to implement this.

what version of exchange and how many users? If its 2003 and not too many users, or the users can actually follow instruction then I would create a contacts public folder, populate it with the required contacts and then assign the correct authoring priviledges to those who will be allowed to update it. Within the users Outlook you can then right click the PF Contacts and from Properties > Address Book specify it to be displayed in outlook as an adress book.