I move adjacent to them and the computer asks if I want to emissary these units. Thinking that I have enough value in commodities on my caravan to win them over, I hit "Yes". Of course I was correct and my emissary was granted. If I had failed, I could have moved piled up some goods on another unit and moved into the same stack as the caravan to try again. Attempts from other stacks are possible but only with goods in that particular stack.