[CentOS] Need help in writing a shell/bash script

Hi Friends,

I am trying to write a shell script which can merge the 2 columns into
3rd one on Centos 5. The file is very long around 31200 rows having
around 1370 unique groups and around 12000 unique user-names.
The 1st column is the groupname and then 2nd column is the user-name.

1st Column (Groupname)            2nd Column (username)
                admin                      ankush
                admin                       amit
                powerusers               dinesh
                powerusers               jitendra




The desired output should be like this

admin:   ankush, amit
powerusers:  dinesh, jitendra


There are commands available but not able to use it properly to get
the desired output. Please help me



Thanks & Regards

Ankush