As with most computing purchases, you will have to balance features and price. If you want an application based on a browser or e-mail, the cost per user will be low. If your users expect more power, you'll have to invest in client software, which will cost more. On the server side, look for a package that runs on many platforms and network operating systems. Depending on the number of users you support, you might have to dedicate more than one server to the groupware application.
Database replication becomes very important here.
Some groupware products have server clustering for good reliability and scalability. The groupware market is evolving fast. You can buy an overall solution in one package or pick components a la carte. The large products such as Notes, Novell GroupWise 5 and Digital LinkWorks usually have modules that support various functions. Many work with third-party products.