Why PACTOR is an ARQ (automatic repeat request) mode: the two stations establish a handshaked link in which the receiving station acknowledges each data frame and the sender repeats anything that arrives corrupted. That handshake is addressed between exactly two call signs, so there is no mechanism for a third station to be added to the link. Others can listen in monitor mode, but they cannot join the QSO; they must wait until the link is cleared and then call.
Watch out Sending a NAK, or any transmission on the frequency, only corrupts the linked pair's frames and forces retries; NAK is part of the internal ARQ handshake, not a way to break in. A steady carrier is deliberate interference and does not create a connection for you.
PACTOR is a handshake, not a roundtable: two stations only. Monitor if you like, but you cannot join.