Is “something” OK “with/by/for” you? To me they are all OK. But technically I think when a person asks (Is “something” OK “with” you?), you're the only person whose opinion is asked. While in (Is “something” OK “by” you?), the opinion of someone else, rather than you, may be important as well.

"Is it okay with you" sounds more correct because since its a meeting the word 'it' refers to a specific thing at the present time, whereas "that" would implicate something which probably is outside the parameters of the meeting that is taking place right now.

I suggest the last sentence... In meetings, you don't often use "is that okay with you" or "is that okay for you"