WP-Members relies on WP’s db schema for users. Users are created using WP’s wp_insert_user() function.
I don’t know what wpforo uses for storing users and/or how users are created, but I would be surprised if it wasn’t similar. However, it may be that for forum access that an additional user meta or user role is used. If that’s the case, then for it to work you would need to create that entry when the user is created. You can hook into WP’s user_register action or WP-Members wpmem_post_register_data to handle adding/updating/removing specific user meta or making changes to the user (such as adding/editing the user’s role).
For wpmem_post_register_data, see: https://rocketgeek.com/plugins/wp-members/docs/filter-hooks/wpmem_post_register_data/
