Seen this a hundred times. You need to open the existing crack enough that you can get some wood glue in there. 2 things can happen. 1, you open it up a little, work some glue in, and clamp it back up or, 2, you split it off when you are trying to spread it out and then you just glue the 2 pieces back together. Pretty simple stuff either way. No need to start over.
Your going to need a table about 8 ft long and 36″- 40″ wide to comfortably accomodate 8 adults, 3 on each side and 1 on each end. An inch and a half thickness of any hardwood will be self supporting for 8 ft. With no additional structural support.
Hairpin legs are readily available commercially. They are relatively cheap and way easy to install but have the obvious drawback of being structurally weak.
I would consider them temporary and, as your skill set improves you can build a solid base with a traditional apron mortised and tenoned into the legs for a permanent “lifetime” solution.