Can regular users (subscribers) still sign up for free on sub sites?

Can regular users (subscribers) still sign up for free on the sub sites if membership is enable on the main site?

I want to enable membership on the main site for selling new sub-sites to sub-site admins. For people who want to sign up as subscribers on each sub site, I want them to sign up as they do on a regular wordpress site, without going through the membership signup interface.

For instance, someone pays me to activate a sub site in my WPMU network. Now he is the sub site admin, on his sub site, he uses Events+ plugin to sell tickets to his customers, which requires his customers to sign up as a user (subscriber) to RSVP and pay. The question is: how can I set up the WPMU and plugin, so when new users – his customers – (subscribers) register on a sub site, they just see the regular sign up interface, not the membership signup interface?

Is it possible?