Hello,the longer Flange distance lens to the shorter Flange distance camera, then could use the infinity focus,but if the shorter Flange distance lens to longer Flange distance camera, then could not use the infinity focus, you can focus on a limited distance.  

There is a Far-Near logo on the focusing wheel. Depending on the viewing distance, rotate the focusing wheel in the corresponding direction. If the viewing distance is far, please rotate the lens to the left. If you are close, rotate the lens to the right. The optimum focus can be adjusted repeatedly according to the actual viewing distance. 

I think it means account email address. They might need to register/sign up as a new user so you can invite and add them using their email account. 

The longer Flange distance lens to the shorter Flange distance camera, then could use the infinity focus.
but if the shorter Flange distance lens to longer Flange distance camera, then could not use the infinity focus, you can focus on a limited distance.
the position of the infinity mark 【∞】of the telephoto lens or super telephoto lens is shifted slightly to the positive side to enable focus adjustment even when focus shift is caused due to temperature changes.The shift range is denoted by an L-shaped line, Be sure to check the focus through the viewfinder even when you shoot at infinity. 

Unlike a tube it won't effect your focus distance nor effect your quality like a doubler. The lens will have more of an effect than anything else. On my Sony a7, I must be sure to use a full frame lens since the sensor is 35mm size