Currently there is no automated way to assign a security role to bulk users. Tried the approach of creating a Team inside the D365 Environment as mentioned here - https://docs.microsoft.com/en-us/power-platform/admin/manage-group-teams#create-a-group-team and then assign a security role to the team, it did not work

There is a MS case with the same # 2112010060002232 and engineer here also says the same that there is no automated way.

For assigning a role for 3K users manually it took me more than an hr. I wish this will come in place so the IT Admin's save a lot of time