The right of your sitting companion (jalis) is that you treat him mildly, show fairness toward him while vying with him in discourse, and do not stand up from sitting with him without his permission. But it is permissible for him who sits with you to leave without asking your permission. You should forget his slips and remember his good qualities, and you should tell nothing about him but good.

http://www.al-islam.org/sahifa/treatise.html

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Note added at 10 mins (2007-07-25 22:44:10 GMT)
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'The example of the righteous sitting companion is like the one who sells misk. Either he gives you some as a gift or you buy from him or you smell from him a good fragrance. And the example of the evil sitting companion is like the ironsmith. Either he is going to burn your clothes or you are going to smell from him an offensive smell.'

http://pixels4hijra.com/blog/wp-content/uploads/2007/05/al-a...
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[From Ibn Hisham:] I was al-Shafi`i’s sitting-companion for a long time, and I never heard him use except a word which, carefully considered, one would not find (in its context) a better word in the entire Arabic language. . . . Al-Shafi`i’s discourse, in relation to language, is a proof in itself.

http://www.sunnah.org/publication/khulafa_rashideen/shafii.h...