How to filter the invitees on the meeting / calls modules?

On the Meetings / Calls module. There is the ability to invite guests. The field is of type participants. If the Related To: is set to Accounts and an Account is select, how can we set the intialFilter for the guests.

I have tried to add an initialFilter in the record.php and I have added a filter on Contacts however they do not take effect.

Thoughts?