How to convert rows into columns using Windows Power shell script? 

Basically I need to join the rows into columns if the first column has the same value. 

Below is the sample input file.
A 1,2,3
A 4,5,6
B 1,2,3
B 4,5,6
C 1,2,3
C 2,3,4
C 3,4,5

Here is the desired output: 
A 1,2,3,4,5,6
B 1,2,3,4,5,6
C 1,2,3,2,3,4,3,4,5

Cheers for any help :-)

For that we would use "Group-Object"