In "Andy Discovers America", When he's telling the story to Opie and the boys about starting the Mayberry Minutemen, Barney says "will you help us". There was another episode where Barney said something similar, I'm thinking "will you lead us/be our leader". Which episode was that?
“Back To Nature” comes to mind as a possibility.
I remember when Opie found fifty dollars and they were waiting a week for someone to claim it but in-case no one did Opie and Barney were looking through a catalog at some toys and stuff and they saw an erector set. Andy told them about when he was a kid he'd had a set of his own and he described a little draw-bridge he built over a small creek by his yard with a cranking mechanism with boats going under it. Opie and Barn were spell bound by the story and Opie said he'd like to build one, and Andy said something like "Sure you can" and Barney said: "Will you help us?" I think was in Opie's Fortune.
Yep! That's it. Thanks.