This is the 757th most frequent Croatian word.
njihovim
"Their" (possessive, dative/instrumental plural form).
Here, 'njihovim' is the instrumental plural form of 'njihov,' matching 'prijateljima' which is the instrumental plural form of 'prijatelj'.
Razgovarao je s njihovim prijateljima.
He talked to their friends.
In this instance, 'njihovim' is the dative plural form of 'njihov,' agreeing with 'idejama,' which is the dative plural form of 'ideja.'
Divim se njihovim idejama.
I admire their ideas.
In this sentence, 'njihovim' is the dative plural form of 'njihov,' functioning as the possessive adjective for 'achievements' in the dative case.