Add basic level to normal multi site sign up


I use pro sites. I want to use the normal WP sign up because it's simple. I also want user to just start with the basic level and afterwards upgrade. How can I add the first level to every normal sign up? If I sign up now I have to manually manage the level of the user.

An alternative would be to select a default level at the pro sites sign up page and hide all levels with css and just show the sign up form.

Let me know what you think is best.