I'm wanting to add a shower to a 1/2 roughed basement. It already has a toilet and sink rough. I'm wondering were to attach the shower drain, and does it need a separate vent or will the vent from the sink work. I know it needs a p trap and it will be 2" pipe. Any other suggestions would help greatly. I will attach pictures.
http://www.dropshots.com/eardled

You need to connect the shower drain between the tee off to the sink and the wye into the main 3"-4" drain pipe. This would be an allowed wet vent.

Or you could cut out a section of the main (it would need to be downstream of the sink wye, since you really don't have enough pipe between the toilet and sink rough for another connection), and put in a 3x3 (or 4x4)x2 wye. Then use whatever fitting you need to connect to the shower location, then a tee which would connect to the wye on one end, the shower drain on the second, and a vent pipe on the third. The vent needs to be at least two pipe diameters (e.g. 4") away from the shower trap, and then you would tie that into the vent for the sink (assuming there are no fixtures on any other floors connected to it) at least 6" above the lip of the sink. This would be a dry vent.