Before adding a new user to a specific hotsite, send us their desired username, email address, and password so we can add them to the system. Once they are added you can now add them as a user to a hotsites and choose their role (subscriber, administrator, contributor, author, etc.)
When the new user is added to the system go to the dashboard of the hotsite you want to give access to. Go to Users > Add New > Enter their email address and select their role to the hotsite > click Add Existing User.
The new user will receive an email asking them to confirm the invitation to be a user to the hotsite.