Find what the user group ID of Administrator group is by running:
SELECT user_group_id FROM user_groups WHERE context_id=0 AND role_id=1;
It’ll probably give a 1 back as a result. Then, you can grant that to user:
INSERT INTO user_user_groups (user_id, user_group_id) VALUES (278, 1);
where 278 is the user ID, and 1 is the user_group_id.