What is the most popular party form?
select * from party_forms group by form_id order by count(*) desc limit 1
Find the emails of parties using this form.
select t1.party_email from parties as t1 join party_forms as t2 on t1.party_id = t2.party_id where t2.form_id = (select form_id from party_forms group by form_id order by count(*) desc limit 1)