Assuming that the border size alternates between rounds and you have 5 people per group it would go something like this:
You are person 1 - make a 12 1/2" square center
Person 2 adds a 3 1/2" border 12.5+3.5(left side)+3.5(right side)-.5(seams)=18.5
Person 3 adds a 6 1/2" border 18.5+6.5+6.5-.5=30.5
Person 4 adds a 3 1/2" border 30.5+3.5+3.5-.5=36.5
Person 5 adds a 6 1/2" border 36.5+6.5+6.5-.5=48.5
Your quilt would be 48 1/2" square when it was returned to you