requesting for logic

There can be multiple ways of doing it but why don't you assign the tasks to the group instead of users and then let them automatically pick based on the availability and the capacity? 

You could just store the last assignee in a table or retrieve it via a query or task report and calculate the next assignee by finding the person in the group after the last assignee.

as the requests were expected in millions on daily basis. so they suggested us to do in round robin method instead of mapping to group

For that, you can create a list of users in a constant or a table or somewhere and can store the active index somewhere. That way, you can always use that index and keep incrementing it. Once the value of it reaches to the count of total users, then set it back to 1

On top of this suggestion i have a qn. actually as of Now i have created the seperate grp for this users . by using the getdistinctusers() i'm getting the users list. if i approached ur suggestion than how can i store in constant or db.

Because we are going to onboard the users in group.

There should be a scheduler or something that would get the users from that group every midnight and update them in the DB. Then you can use that as a list. 

We can directly work with the group as well but I am not sure if we will get the users always in the same order.