This means that they can start to have thoughts about offending behaviour which are reinforced by other offenders.
If paedophiles had access to non-sex-offenders to talk to they'd get early nudges about how unacceptable their thoughts are, and guidance about avoiding those thoughts and behaviours.
I'm not sure how the "friendship circles" worked out. Maybe they were hopeless? (http://news.bbc.co.uk/1/hi/england/berkshire/3873083.stm)