A group of people, led by a man with a backpack, are walking past people sitting on the grass in front of an old building.
The person walked by the group of people standing in the mud.
A person implies only one person, which is not a group of people.  One cannot be sitting and standing at the same time.  Mud is not grass.
exists e3 e1 e2 s1 s2 x1 x2 x3 x4 x5 x6 x7 x8 x9 x10.(n_grass(x8) & Theme(e3,x6) & Actor(e3,x1) & v_walk(e3) & in(e2,x9) & of(x9,x10) & n_building(x10) & a_old(s2) & Theme(s2,x10) & n_front(x9) & on(e2,x8) & Actor(e2,x7) & v_sit(e2) & (x6 = x7) & n_people(x6) & a_past(s1) & Theme(s1,x6) & Actor(e1,x4) & with(x4,x5) & n_backpack(x5) & n_man(x4) & Theme(e1,x3) & v_lead(e1) & (x1 = x3) & of(x1,x2) & n_people(x2) & n_group(x1))
exists e2 e1 x1 x2 x3 x4 x5.(n_mud(x5) & n_group(x2) & n_person(x1) & by(e1,x2) & of(x2,x3) & in(e2,x5) & Actor(e2,x4) & v_stand(e2) & (x3 = x4) & n_people(x3) & Actor(e1,x1) & v_walk(e1))