How do I allow my WPMU users to create more than one site ?

Quick question : I have a WPMU/Buddypress website, and when they register my users have the ability to create a website with their account.

However, when they did that and they are in their dashboard, they don't have the possibility to create other websites :

If I remember correctly, there should be a "Create a new site" function somewhere on the top bar, but there is not that on my website.

How can I get that function back, so that my users can create more than one website ?

Thanks a lot, regards,