Right now, I am in the meeting, please don't disturb me.
Right now, I am at the meeting, please don't disturb me.
The both `in the meeting' and `in the meeting' do not make any difference as to meaning, I think.
Could you help me on this?
I would say "in a meeting."
If there were a multi-day meeting being held off-site, you could be "at the meeting." But to just mean that you are in a conference room for an hour or so, say "in."
(American English perspective.)