[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