Imaam’s wudhu breaks and if 2/3 people are reading namaaz

If two people are making namaz with jamat one is the imam and the other muqtadi. How will they both stand, side by side or at the back of the imam. Suppose a third person comes and joins them the how will the 3rd person stand will he stand at the side or muqtadi or at the left side of the imam or at the back. If he stands at the back he will be standing alone. I have heard that he will pat on the back of the muqtadi and then he will move backward. Is this the correct procedure. How will the muqtadi know that for what reason one is patting him. Moreover he will move backward while remainging in the position of salah or break the salah and come back.
How will the muqtadi know how many feet to move back ward. These are troubling me as this thing has happened before also. So please give me the correct ruling.
Jazzak Allah!