Logging into subdomain wordpress site

I’ve got a wordpress site, ex: https://www.mysite.com.

What I’d like to do is to have a user be able to log in to the site and get “protected” content on a subdomain. When the user comes to the main site, they get info, a sign-up form, and a contact form. When they log in, it goes to a subdomain (ex: members.mysite.com).

I can (easily) do it if I install 2 versions of wordpress, one for the main and one for the subdomain, but the client would like to be able to use only one version of WP and be able to edit everything in one control panel.

Anybody got any ideas on this?

Thanks.