An error occurred at element Add_User_to_All_Chatter_Group (FlowRecordCreate).
INSERT --- INSERT FAILED --- ERRORS : (MIXED_DML_OPERATION) DML operation on setup object is not permitted after you have updated a non-setup object (or vice versa): CollaborationGroupMember, original object: User
I am assuming during creation, the user id is not yet committed, so it not able to set variable in Flow.After do some googling , I found some solution that can be applied on my case.
I also add new Record Lookup in my Flow, to check if the user already exist in Chatter group and Decision when the Lookup Result is not found.
In Process Builder , I use Scheduled Actions so I have to use 'Formula evaluates to true'.Scheduled Actions option is not available if 'Conditions are met' is ticked.In order to run this process builder as soon as possible,I choose 1 Hours Before LastModified.
After that, I create new user and there no more error.Also working fine for update.
Have a nice day.