The said Eliza, John, and Georgiana were now clustered round their mama in the drawing-room.

这个句子使用clustered没有错误。cluster作为及物动词的用法多数词典未收录，但《韦氏词典》有及物用法。本句中clustered为及物动词的过去分词作表语，句子为系表结构。

COCA语料库有大量类似例句如：

The house servants were clustered outside the dining room to eavesdrop,...

Fourteen people were clustered around the bomb when it detonated.

A few people were clustered together outside the front door, smoking and talking beneath a protective overhang.

The pearl divers were clustered at the end of the beach where a rocky cliff rose against the sea.