The helper class method used by both of these implementations that creates and posts the GroupMembership instance is not correctly populating the GroupMembership resource - dirty properties are not being set, and as of 0.9.0, the datastore ignores sending non-dirty properties.

Resolution: construct a GroupMembership instance and call setResourceProperty for both the ACCOUNT and GROUP fields so that the underlying implementation can 'dirty' those properties so the data store knows to include them in the request body.