I have

name register_date
Foo 2013-10-10 16:54:10
Foo2 2013-10-11 16:54:10
Bar 2013-09-10 16:54:10

I want

month count
2013-10-01 2
2013-11-01 1

SQL

SELECT COUNT(*), DATE_FORMAT(register_date, "%Y-%m-01")
FROM register_users
GROUP BY DATE_FORMAT(register_date, "%Y-%m-01")

References