What is the name of the effect whereby socialising in a group reinforces group beliefs?
There is also the related phenomenon of 'Group Polarisation' (see Myers and Lamm, 1976; Isenberg, 1986), where groups are found to make more extreme decisions and hold more extreme opinions than its constituent members. Not sure if that's specifically related to what you're looking for but I think it's important to keep in mind.

References


Isenberg, D.J. (1986). Group polarization: A critical review and meta-analysis.. Journal of Personality and Social Psychology, 50, 1141. PDF
Myers, D.G. &amp; Lamm, H. (1976). The group polarization phenomenon.. Psychological Bulletin, 83, 602.