Can I create a sub-team project in TFS?

In TFS, I know I can create several team projects under a team project collection. Now, based on my team, we want to create sub-team projects, can I do that using TFS?

I found a link http://msdn.microsoft.com/en-us/library/vstudio/dn306083.aspx which introduces the agile portfolio management to support backlogs across multiple teams. Do I think that the agile portfolio management can implement the sub-team projects?

Another question: what does "Portfolio" mean?

Thanks,

shmily