Can I create a user on a subdomain membership from a main sales site OPM

If you're looking to create a user from an OptimizeMember installation on a sales site and add them to a membership on a different domain or subdomain:

I'm afraid each Wordpress site you have, including subdomains are treated as individual sites, so you cannot allow a signup from one site to register on another site with our default registration processes.  
There are a couple options you could consider:
1) Create your order pages on the subdomain site and then redirect to these sales pages or order pages from your sales pages on the other main site.  You would then create the order functionality on the same site as your members area which would work fine
2) Use this tutorial steps: https://optimizepress.zendesk.com/hc/en-us/articles/201128708-How-to-create-a-URL-order-link-for-Paypal-Clickbank- - to create a button URL which can be used on a different domain.  This is a little more complex but is possible if the steps in the video are followed carefully