WP-Client will work on Multisite installations, but does not share users, files, HUBs or Portal Pages across the entire network. For example, if you create a client on Site B, that client will not be able to also login with the same capabilities at Site C. However, with WP-Client installed on Site B, the plugin will function as intended on Site B only, just like it would on a normal standalone installation of WordPress.
Each unique URL that contains a site ( alpha.maindomain.com, bravo.maindomain.com, charlie.maindomain.com ) will require a separate license of WP-Client in order to install on that URL. Meaning, a Single Site WP-Client license does not automatically entitle every site on a multi-site network to install/use WP-Client. WP-Client cannot be activated network-wide. Each separate multi-site domain would need to be listed individually under your authorized domain.