Suppose there are X people in two groups and each person digs Y holes, then 1 group has X+ 1 person, each person digs Y+ 1 hole, and 3 groups have X-2 people, each person digs Y+2 holes.

If 1 group digs 18 more than 2 groups, we can get (x+1) * (y+1)-xy =18.

If two groups are dug less than three groups, we can get (X-2)*(Y+2)-XY=2.

X = 10 and y = 7 can be obtained from the above two formulas.

So two groups of *** 10 people, each dug an average of 7 pits, ***70 pits.