Hi I would like to view the description fields
and complete description
in the group details
what file I have to change ?

Hi,

sorry, this topic not clear for me.
you can make group detail as default tab by visiting jomsocial backend > theme designer > groups
and group description always showing complete description (not truncated)

and for the file located at:
/components/com_community/templates/jomsocial/layouts/groups/single.php

and you can changing it by using override method:
documentation.jomsocial.com/wiki/Customizing_Template

so the changes not disappear at upgrade process.

Regards