Please list some good examples for those of us with poor knowledge of the subject.
Hmmh. There are many characters but I don't remember if they all hold land and if they do, where they are. At least the following have land in 1066-1081:
Georgios Palaiologos - the son of the starting ruler of Epeiros. Georgios was a very loyal ally of Alexios and instrumental in his rise to power. He also provided tactical insight to Anna when she was writing the book, describing Alexios' battles in great detail.
Nikephoros Bryennios - Usurped the Imperial throne in 1077. Considered the best tactician of his generation. Defeated by Alexios shortly before his rise to power and blinded. Continued in Imperial service despite his blindness. His son, also named Nikephoros, married Anna Komnene.
Robert and Bohemond d'Hauteville - Not Byzantines but featured very prominently in the book. Bohemond in particular is virtually the second protagonist of the book, described as a passionate master of intrigue with an undying love for waging war against Alexios. One of the few 'latins' for which Anna holds great respect despite his endless disturbance of peace.
Romanos IV - While he isn't featured in the book, his sons Leo and Nikephoros are. They served Alexios extensively, though Leo died in 1087 and Nikephoros several times attempted to assassinate Alexios for which he was blinded and exiled.
Theodore Gabras (Trebizond) - A very skilled commander who defended Trebizond against Seljuk hordes. Revolted against Alexios but was reinstated because of his skill.
That's some at least. There are also all the people who usurped the throne in post-Manzikert chaos. That is, Nikephoros Botaneiates, Konstantinos Doukas and Alexios himself.