Wanna tame it in multiplayer first have pt or arge ton of Trang arrow or Tranq bullet have a tribe mate make him pick you with the pt or arge then shot the bullet on the face till it knock down feed it tons of prime meat
The Rex are very slow . Put a stone gates around him while he is stuck and use a bear trap then put the x3-x4 stone gates then tranq him out . Make sure you clear the area before setting the bear traps others creature can spring it and u would be left with using your self or a tanky dino for a distraction
If you use the bear trap strategy which is if you lead it into a Dino gateway that is shaped in a triangle and with a bear trap in the middle (Large Beartrap) and when it is stuck you must put a dinogate behind it to trap it and then knock out.