Sorry, I am not sure to understand what you are trying to do.
What information of a user are you willing to access?
If you need information concerning its account, you will need to get the user as I described just before.
If you need to access their user timeline, you don't need to create the user, you can do it directly from their user id.
var tweets = Timeline.GetUserTimeline(userId);
In fact with Tweetinvi you should always be able to do an operation related with a user from its user id.
Please let me know exactly what information you need and I will let you know how you can access them.