To sign in to their Exhibitor Plus account, your exhibitors use the same URL you use to sign in!
Once they log in, they will be able to select your show in their Active Exhibitor Accounts
Path: Exhibitor Dashboard> My Exhibitor Profiles
From here, they can edit any exhibitor information you've given them access to, or even purchase additional booth personnel slots and Sellable Services (if you've enabled these features for them.
Path: Registration & Orders> Registration Settings > Optional Settings, Exhibitor Plus Settings