Okay, I added this myself. For the old communities I did this:
db_foreach communities {select community_id from dotlrn_communities} {
set members [dotlrn_community::get_rel_segment_id -community_id $community_id -rel_type dotlrn_member_rel]
set public_folder_id [dotlrn_fs::get_community_shared_folder -community_id $community_id]
permission::grant -party_id $members -object_id $public_folder_id -privilege write
}
