Users can be members of many groups at a time, however, a user can only be active in one group at a time - this is their 'Default Group'. User can specify this group using the Default group control under the My Account page, any data imported by the user will be imported into this specified default group.
For non-admin users the 'Default Group' is set via the 'Administration' link ...'My account'. For admin users it would be 'Administration' link ...'Users & Groups' ...'My Account'.