会议进行中 = Meeting in progress If you wish you can add 'Don't disturb' You can write 'occupied' if you want to say this room is reserved for a meeting and not available to others. But the word would be easily associated with occupied toilets. 'Meeting in progress' is best.

I find some companies they use the sign "Occupied/Vacant" or "In-use/Vacant".

你好Fiana, Are you going to write it in chinese or english? Froopy's answer would be sufficient if you're going to write it in english. Just to add up, you can also write "meeting on-going" or "session on-going". I'm not sure though how you write it in chinese. I do not suggest you to wirte neither "Meeting Process" nor "In the meeting" in literal english. It may imply a different meaning. I hope this helps too... 谢谢。。。

If you are trying to show that the meeting room is being used, and therefore is not available for others, you would say: "meeting in progress" (which means that a meeting is currently happening) or "meeting room in use" (which means that the room is unavailable, regardless of why it is busy).