MNI Error-How to generate Multiple groups with Group Id's that need to store in DB?

Pass in an array by reference and append the group ids within the sub process and then persist the group ids in the DB.

[View:https://docs.appian.com/suite/help/23.2/Sub-Process_Activity.html#passing-a-process-variable-as-a-reference]

What do you mean with "multiple id's are not getting generated"?

You don't have to use MNI for this. Next, IDs generate automatically in the DB. You don't generate them before sending data into DB. 
Just provide your CDT,= and Write to DSE smart service will write your data, and will give you the IDs. Make sure you have kept the WTDSE's Input-> Save Into blank and Output -> Stored Values as the same CDT.