Please check if
1. windows sign-in is enabled in Menu > Active Directory
2. users are using their user-ids and not their email addresses to log in
3. proper domain or sub-domain is added to the “Domain” field in Active Directory configuration. Global domain names should not be used. For example, it should be “net.prolab.com” and not “prolab.com”
4. User is not part of a nested group. If you have added an AD group, it should have the users who will be accessing Prolaborate. If the Users of groups inside this group try to log in, they will not be able to log in.