Only the Imaam and one muqtadi are in the jamaat.

Answer

If with the Imaam there is only one Muqtadi, and another one arrives to join the Jamaat, then the actual mas'ala is that the newcomer should tap the muqtadi on the shoulder or slowly pull him backwards to form a saff behind the Imaam. If the muqtadis are not aware of the mas'alah, the Imaam himself should take one big step forward to be infront of the two muqtadis.