sendGrid[rowFromFirstGrid, 3].CellValue = true;
If you will explain more aabout what you are working with, then maybe we can suggest something more specific.
// assume selectedGroup is the value you want to compare, groupColIndex is the column holding this value and checkCol is the column index of the bool value you want to set. for(int row = 1; row <= grid2.Model.RowCount; ++row) { grid2[row, checkCol].CellValue = grid2[row, groupColIndex].CellValue.Equals(selectedGroup); }