A hub is a way to group contacts who may want to frequently message one another or share files and/or photos with other members of a Hub.
Contacts in a Givebacks (formerly MemberHub) site can be assigned to as many Hubs as you want and allows contacts to join groups or classrooms in which they are involved. Use Join Hubs page in Communications to choose hubs to join. Additionally, admins may add contacts to hubs.
Hubs are a great way to allow contacts to communicate within their hubs, but limit access to communications with the entire organization. Hub admins are allow to message, create events, and upload files within their hubs. Hub contacts will receive the messages, events and access to the files. Depending on the Hub settings, the Hub contacts may also be able to create messages and events and upload files.