how would I send a monthly email automatically
You could use the CiviRules extension to accomplish this in combination with the Email API extension. Both extensions can be found in the extension directory. 

In CiviRules the DailyGroupMembership trigger does what you want, it only does it on a daily basis rather than once a month. You might either come up with some CiviRule conditions to make it run only once a month or even better, adapt the DailyGroup trigger to allow a selection of day/week/month. 

Some examples of CiviRules can be found here: https://wiki.civicrm.org/confluence/display/CRMDOC/CiviRules+Cookbook