I want one word substitution for it. I thought intruder but it is incorrect.
That would be: Eavesdropper
The verb eavesdrop is to: "Secretly listen to a conversation"
(You didn't explicitly state that they were doing it surreptitiously but I assumed that it was implied by the nature of the question.)
If you want an informal word for this you could try earwig. In this context earwig is normally a verb:-
- to eavesdrop [Collins English Dictionary via the Free Dictionary]
but I have heard many times the person doing this referred to as an earwig.
Depending on context, consider;
prying ears Ngram
Carmen shrugged at Katie's questioning look, and stepped through the door, closing it against prying ears. Your Dictionary
listen in (on)
: to listen to a conversation between others; eavesdrop. AHD
Bystanders are those who are nearby, overhearers are possible interpreters and these include listener-ins (those that are attempting to listen) and eavesdroppers (those that are secretly listening) Google Books
be a fly on the wall
: to surreptitiously listen to another person's conversation Quotations Link
: (n.) an act of secretly listening to or recording a person's telephone or Internet conversations, often as part of a police investigation.
: (v.) to listen in on by means of a wiretap. The Free Dictionary