I need help adding multiple users to multiple groups.  The caveat is that my source data has ONE group per row with all users in corresponding row.  

Three questions

1. Which bulk operation would I use to achieve this?

2. What would my CSV row headers look like?

3. What format does the data need to be for the corresponding headers Ie, Groups - "CN=Group1,OU=Groups etc.."

Example

GroupName      Members

Group1            Joe, Jimmy, David, Sammy

Group2            Joe, Jimmy

Group3            Jimmy

Group4            David, Sammy

I might be able to extract the source data in this format although it will take some heavy excel manipulation to achieve

SamAccountName      Groupnames

Jimmy                        Group1, Group2, Group4

Joe                            Group1, Group2

David                         Group3, Group4

Sammy                      Group1, Group2, Group3, Group4