Can users be part of the design team?
Ah, the "User" question. Should the people for whom a product is being designed be part of the design team? The politically correct answer is to say "yes, but of course." But I'll take the other route: My first thought is why would I want to do that? What do they know about design? They probably don't even understand their own true needs. (But before you get too riled up -- read on.)
The people who use your product are not usually amateur designers. Moreover, as is well documented in the psychological literature, people are not conscious of the reasons for many of their actions. They do no realize the influences upon them, both external and internal. They are apt to describe the symptoms of their difficulties and request solutions to those symptoms, whereas the better solution is to get at the true needs and the root causes of problems.
On the other hand, if we ignore the users of today's products, disaster is apt to result. Many a system -- usually expensive ones at that - have failed because of inappropriate attention to the needs of the people they were intended to serve.
So, the answer to the question is complex. In part, it depends upon the nature of the system being designed. Designers must have a deep understanding of the system under construction and of the needs of the people who are to use it and benefit from it. What do these people know that is so special? Can it be quickly learned by the designers, or does the knowledge take years to acquire? If the knowledge can be quickly acquired, stick with the design experts. If the knowledge takes years to acquire, then by all means, hire one or more and use them as expert informants. Note the word informant: this is not the same designer.
Everyday people are not very good designers. They don't know what is good for them -- or bad. Their opinions are, well, opinions. The designs they produce are apt to be klutzy, confused, crowded. I prefer design by experts - by people who know what they are doing. Knowing how people will use something is essential. Knowing about the people is essential. But asking their opinion is ... (I almost said that it is like expecting the average citizen of a country to make an informed, intelligent decision about who should be the leader of the country, but I decided not to say that).
But then again, ignoring potential users is guaranteed method for failure. So, yes, users must be studied, consulted, and listened to. But should they be a part of the team? Ah. As I said above, if the system is complex enough that the skills required to work with it take years of experience, then yes, hire them as expert informants. Otherwise, no, good designers can acquire the necessary skills.
A comment on the word "user." Horrible word. Someone who uses? Although it is not nearly as degrading a term as consumer": someone who consumes. Nor is it as bad as the term experimental psychologists like so much: "subject." Why not call them people?