We had this same trouble. We never found a solution to include multiple parties in the To line but we can include many in the CC line. We placed the following in the Action as 'Go to URL.'

="MailTo:user.one@email.com" & "?cc=user.two@email.com;user.three@email.com"

However, in order to stop confusion we found that we could leave mailto blank and put all recipients in the CC line. This helped us politically.

="MailTo:" & "?cc=user.one@email.com;user.two@email.com;user.three@email.com"