Im making a webpart where a user should be able to add other users from active directory, without being able to know their full name etc. The field should be autocompleted in some way (like googles searchfield for example). I have seen examples of this on ordinary websites but will I be able to do this on a custom sharepoint webpart and how do I do it?
This sounds like a case for the Sharepoint PeopleEditor control. By your description I think it will satisfy all your requirements.
Karine Bosch has a nice blog post about the user control here
I see you tagged this question with jQuery and ajax, so I assume you want to do this client side (on the browser)... IF you are open to using SPServices library (http://spservices.codeplex.com/), I have created a People Picker widget that you can use in your custom UI... More info. on it here: http://purtuga.github.com/SPWidgets/