Membership fees and wordpress themes

In terms of the licence for WordPress itself, you can do pretty much whatever you want with a self-hosted site.

That said, it will depend on the terms of service of your host, as well as any particular licence restrictions that may or may not be imposed by the creator of the theme that you are using.

You’d be better off getting in touch with the theme’s creators directly to ask this question – it’s too general to be able to respond to here!