John Gill Commentary Nehemiah 6:18

"For there were many in Judah sworn unto him, because he was the son-in-law of Shecaniah the son of Arah; and his son Jehohanan had taken the daughter of Meshullam the son of Berechiah to wife." — Nehemiah 6:18 (ASV)

For there were many in Judah sworn unto him
To Tobiah, who not only in a private manner corresponded with him by letters, but bound themselves by an oath to him to be true to his interest, and do as he should advise them:

because he was the son in law of Shechaniah, the son of Arah ;
of a family that came up with Zerubbabel from the captivity, (Ezra 2:5) and very probably of considerable note:

and his son Johanan had taken the daughter of Meshullam the son of
Berechiah ;
a very eminent person, concerned in building the wall, (Nehemiah 3:4Nehemiah 3:30) .