I think the only way to keep feds from infiltrating groups is to be decentralized. Your only group identity would be a shared ideology. There would be no hierarchy, there would be no ranks. It would be organic, and there would be a shared banner to fight under when things really hit the fan. But to try to gang up and enact change as a group in modern-day America is only asking for trouble, and you will be flooded with feds from top to bottom.