The receiver's partner should not crowd the "T" excessively.
However, he/she should be about a foot or two or three down the service line from the "T". Perhaps a foot or so back from the service line. My usual position is for my lead foot to just be touching the service line.
First responsibility - help on the service line call. watch the net man out of your peripheral vision.
The instant you know the serve is good, immediately turn your focus to the net man. He is your sole responsibility right now. You at this point are a Yankee shortstop. Knees bent, elbows in front and this is the one time it might not be a bad idea for your ready position to be racket lower than normal volley position (makes 1/2 volleys easier and less likely to pop up). If the net man gets to (cuts off) the return, your job is to keep the point going by reflexing a half volley or volley back in play. If your partner tries an alley shot on the return and the net man gets to it, one of the most likely plays is a short volley dropped over the net in the far alley. (The sharply angle volley than parallels the net is far less likely) Part of your responsibility is to scramble over and get that shot. If your partner's down the line return is so weak and slow that the net man can crush it on that super sharp cross court angle or can whack a volley at your feet that you cat handle, the problem is not your positioning and/or skills, it is your partner.
IF the return is not going to be cut off by the net man (You see he is not moving to the ball, placement of the serve and resultant return makes it extremely unlikely that he is going to be able to cross and cut it off, receiver can and does hit a sharply angled low return (KEEP THAT PARTNER!!!) or the like, then you go to step 3.
Step 3, it is extremely difficult for server coming in to get all the way to the net and crack an offensive volley either down your alley or into the middle. So now you have the chance to make that quick transfer from defense (which you have been on) to offense, (which you want to be on) A 45 degree turn and diagonal hop (towards the net and towards "your" alley to position you more centrally in the service box on your side) to face the server's reply to the return. IF the server hits his first volley a little high, or right at you, you are in good position.
And THAT, my Ducks, is why you start near the "T", but not right on it. At least at rec level tennis!