If you're looking to join a big market, I'd recommend you look at the larger federations. They normally have their own markets.
However, if you're looking to grow your economy, joining a common market doesn't really help. It won't increase profits, as contracts are not more profitable than the open market.
What are you hoping to get out of joining a common market?
My two cents: If you are a new player with a goal of economic gain, I think you'd have more success by focusing on trying to improve the profitability of your corporations than spending time thinking about your market.