After a post office is upgraded to GroupWise 18, users who have accounts in that post office can start using the GroupWise 18 clients. You can give users access to a GroupWise 18 software on the GroupWise download page, so that they can install the GroupWise client themselves, or you can use the other methods of client software distribution described in Distributing the GroupWise Client
in the GroupWise 18 Administration Guide.