Scenario -> Creation Contact share record and in the same transaction creating GroupMembers for one of groups related to custom object.
Insert failed. First exception on row 0; first error: MIXED_DML_OPERATION, DML operation on setup object is not permitted after you have updated a non-setup object (or vice versa): GroupMember, original object: ContactShare: 
As per doc
Mixed DML only comes up when we we mix setup and non-setup objects in a transaction.
So isn't both
GroupMember setup objects?
I know how to fix this, but got confused with error message as I thought we can update multiple setup objects in one transaction.