clause with who/that/whom

[Saneta]

1.,, The people who work in the office are very nice.''
Is there a possibility to write the above sentence in this way:
,,The people that work in the office are very nice.'' ?- my grammar book doesn't show this possibilty, I don't know why because I rather think it is correct.

2. ,,You invited some people to your party. Some of them couldn't come. You tell someone:
Some of the people I/we invited to the party couldn't come.
or
Some of the people who/whom/that we invited to the party couldn't come.''

Can I also say:
,,Some of the people who/whom/that I invited to the party couldn't come.'' -There is not this possibilty in my grammar book and I don't know why...

Thanks!

[Mister Micawber]

1. 'That' is often used informally with humans, but 'who' is preferred.

2.
Some of the people I/we invited to the party couldn't come.-- The usual way.
Some of the people who/that I/we invited to the party couldn't come. -- Acceptable informally.
Some of the people whom I/we invited to the party couldn't come.-- Very formal.
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Native English teacher at Mister Micawber's