Echo that a user experiences is determined by the hardware(speaker and microphone) and the browser(chrome/firefox).
Some of the reasons that a user might experience echo issues are -
1) There are two publishers in the same physical room or
2) Two publishers publishing from the same machine.
Below is a list of things that the user could try to reduce echo
1) Reduce the volume of speakers
2) Reduce sensitivity of microphone. This will require a change in settings
3) Use Headphones
To determine the cause of echo, users could try the following techniques -
1) Join the session from a different computer. If the echo has reduced, then we know that there is an issue with the hardware.
2) Join a different chat room. If the echo has reduced, then we know that there is an issue with this particular chat room caused by a different user. In this case, selective muting (muting people who are not talking) can help.