Cannot add people to site collection using WSS 3.0

I'm trying to add some people to a site collection created through the New Team Project wizzard, from Team Foundation Server, but I'm unable to find anyone in my the list (which should be populated with AD users) other than the first site collection administrator and the second administrator by using the people picker. What should I do?

I suppose it's also woth mentioning that if I use the people picker from the SharePoint Central Administration, I am able to see all the users in my AD.

Thanks in advance!