“When there are three people, two should not converse together at the exclusion of the third.” (Adabul Mufrad: Hadith #1166 pg 722)
This is straight forward, two people should not exclude the third and speak because this will make him feel unwanted and lonely. Sometimes, two people sit together and begin a conversation in such a manner that the other feels that they are talking about him. The behaviour of a group of Muslims should be such that they make everybody feel wanted. In the modern day, this can also apply to the usage of mobile phones. For example, if two people are sitting and one of them gets a phone call, he should not have a half an hour discussion with the person on the phone and discard his friend in that period. Rather he should host the person whom he is with.
In the same manner, it is not permissible for a person who sees two people talking and then butts into their conversation. Once Abdullah ibn Umar (may Allah be pleased with him) was speaking to a man privately when Sa’eed al Maqbari (may Allah have mercy upon him) entered upon them. Ibn Umar pushed him away and said, “When you see two men talk to each other, you should neither stand near them, nor should you sit there unless you have their permission to do so.

