I think most of us are here now. Let's get started.
I think most people have arrived by now, so maybe we should begin.
この場合、次のような言い方ができますよ。
ーI think most of us are here now. Let's get started.
「ほとんどの人がここにいると思うので、始めましょう」
ーI think most people have arrived by now, so maybe we should begin.
「もうほとんどの人が来たと思うので、始めましょうか」
ご参考まで!