Hi ratuspro.
CreateAllianceRequestAsync should be called by the group owner, and only the group owner is able to assign roles to members.
It is not possible in the current version of SUGAR to set the default permissions assigned to a user when they join a group. only the group admin (the one who created the group) can assign these roles. Setting default permissions for relationships is a feature in the pipeline, which will not be available in the game jam version.
Hope this helps!