Does it matter?

Salam aleikom,

Let's say someone in a prayer room is praying a sunnah prayer. Another person enters the room and believes that the person who's already there is praying the fard (obligatory) prayer and joins the prayer. Will his/her prayer count?. Does it even matter what prayer each person is praying if they pray in congregation?. I'm thinking of putting a sign in the prayer room which says something like "Sunnah prayer going on" and the other side will say "Fard prayer going on" and put it on the bench so everyone entering the room knows what prayer the first person is praying.