In WordPress you can set this in Settings > General in the WordPress backend in the New User Default Role setting.
How do you add a site role to a person? Example, how to make 1 person have 2 or 3 site roles?
Well I don’t entirely understand where you are getting at, but I mean its possible for a user to have 3 different roles.
For example.
Site Role – Subscriber – Default WordPress role
Forum Role – Participant – Default bbPress role
Membership Role – Premium Member – Some paid membership framework plugins role
If you want 1 custom site role with different capabilities that other roles might have, consider creating a custom user role. But be aware of what each capability does, so that you will not give your users permissions that you do not exactly want them to have.
Creating a custom bbPress role guide.
https://codex.bbpress.org/custom-capabilities/
bbPress user roles and capabilities
https://codex.bbpress.org/getting-started/before-installing/bbpress-user-roles-and-capabilities/
WordPress roles and capabilities
https://codex.wordpress.org/Roles_and_Capabilities
And a plugin I would recommend to create a custom user role with custom capabilities would be a plugin called Members by Justin Tadlock.
