You could add a trigger into your database, automatically adding more/other groups on creation of new clients. Note that, however, this change may be in conflict with future Pi-hole updates so you'll have to make sure to reset your local changes before updating (it will just not work, otherwise, there is no other harm involved here).
The relevant triggers you'd want to look at are:
(located on your system in /etc/.pihole/advanced/Templates/gravity.db.sql
)
They automatically add the group 0
(the default group) on every newly added item. You can modify the triggers to add more group links, such as:
CREATE TRIGGER tr_domainlist_add AFTER INSERT ON domainlist
BEGIN
INSERT INTO domainlist_by_group (domainlist_id, group_id) VALUES (NEW.id, 0);
INSERT INTO domainlist_by_group (domainlist_id, group_id) VALUES (NEW.id, 1);
INSERT INTO domainlist_by_group (domainlist_id, group_id) VALUES (NEW.id, 2);
END;
The changes should be absorbed by a pihole -g
run (I have not tested this, but I'm fairly confident).