Who are the masters and the slaves in Glubbdrubdrib?

From Swift himself;

Glubbdubdrib, as nearly as I can interpret the word, signifies the island of sorcerers or magicians. It is about one third as large as the Isle of Wight, and extremely fruitful: it is governed by the head of a certain tribe, who are all magicians. This tribe marries only among each other, and the eldest in succession is prince or governor. He has a noble palace, and a park of about three thousand acres, surrounded by a wall of hewn stone twenty feet high. In this park are several small enclosures for cattle, corn, and gardening.

The governor and his family are served and attended by domestics of a kind somewhat unusual. By his skill in necromancy he has a power of calling whom he pleases from the dead, and commanding their service for twenty-four hours, but no longer; nor can he call the same persons up again in less than three months, except upon very extraordinary occasions.