This is a bit more difficult.
When it comes out of the recordset, you will need to convert the coma separated list back into an array using the explode() function, and use the in_array() function to determine in the checked value for each check box is in the array.
For example:
<input type="checkbox" name="CheckboxGroup1[]" value="1" id="CheckboxGroup1_0" <?php if(isset($row_RecordsetName['ColumnName']) && is_array($row_RecordsetName['ColumnName'])) {
if(in_array("1", $row_RecordsetName['ColumnName'])) echo("checked=\"checked\"");
}
?> />