The basic principle is that the group that is most often in contact with a person will also be the group most likely to be aware if there is a change in contact details. In some cases the person in question will be working equally closely with several groups, and in such cases this will have to be resolved between the groups during the interim period. However, some roles naturally suggest that the person should have that group as primary, such as 'ME', 'Staff' or 'Co-ordinating Editor'. Other roles will typically indicate a closer affiliation, such as Editor. Editor would therefore take precedence over the role 'Author', which in turn is a stronger affiliation than 'Mailing list', etc.
See also Primary group.