how to create a selection group in edit mode? it is pretty annoying to work with different meshes as the same object.

Is there any way I can define a specific numbers of vertex/edges/faces to a group so I could just click that group to all?

I hope I was clear

thanks
Yes, you can define vertex groups per object: 




In Edit Mode move to Properties Editor > Data > Vertex Groups
Hit the + sign to create a new vertex group (and rename it to something useful)
Make a selection and Assign the desired vertices to that group


Now by using Select the vertices will automatically selected in the 3d viewport.