Hi Gayatri
If customer has single authentic source for maintaining user information (ex. IDM , hr system or active directory) then required user data can be pulled from that system to determine all active users in the organisation to develop chat application you mentioned.
I believe user would atleast have hr system (SAP HCM, people soft , success factor etc.) which can be used as authentic source of system to determine user information.
As to develop chat application you would not need to have users authorization data so IDM may not be required.
Let me know if you have further questions.
Regards,
Pradeep