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We only provide search for group name, not search content within group :) and It's not possible in this version of JomSocial. However, you could suggest it for the future versions in our uservoice:
uservoice.jomsocial.com/forums/101561-feature-request
and if you're in a hurry, you could have oDesk developers custom-code a solution for you:
jomsocial.com/jomsocial-developers
:)