Dealing with feedback is challenging, as the reasons are numerous, but not always obvious. Below are some common causes:
Headphones are too loud
Using Safari as it has poor AEC abilities; use Chrome instead.
&proaudio or &stereo mode is being used. This mode will disable echo-cancellation and so you must use headphones in this mode.
Screen-sharing the desktop /w audio capture on, especially in the case of a group room, will create nasty feedback for others.
Incorrect OBS configuration is common, especially if the echo is only heard in the RTMP broadcast or recording, and not by those using OBS.Ninja themselves
Having two browser tabs open (such as one with the Youtube output playing) will cause echo. Echo cancellation only works within the same tab that the audio is played back and captured, and only if the echo is not prolonged.
A good way to troubleshoot is to mute one person at a time in a room, seeing if muting any specific single person solves the issue for everyone else.
Normally the person who isn't hearing any echo or feedback is the cause.
If you identify that person, triple check that they are using Chrome and not Safari, make sure they are wearing headphones and that the audio is correctly playing into them, and have them close all other browser tabs and applications.
If the issue is only within OBS, this is likely an issue with OBS and not OBS.Ninja. Try disabling all global audio devices, muting audio devices in OBS one at a time, and double checking the advanced audio mixing settings.