Hi Angie,
You're right. The Portal will authenticate its users via AD or LDAP according to what you chose to install.
The AD setup is quite easy, just create a bunch of users on the Active Directory for Windows.
LDAP is easy too, typing LDAP strings with user names and domains.
I don't know from what Tools Release LDAP support began, I just tested it on E810 8.94J1 and it worked.
For Portal you need WebSphere Portal, IBM LDAP Director and to install a local database (UDB, SQL or Oracle).
Sebastian