The easiest is to just go to the SEND button and choose the option "MESSAGES".
This will show a popup where you can choose the group you wish to send to.
"Groups" can be e.g. "delivered", "viewed", "accepted" and "declined".
You can choose to send to one group or to select multiple groups and send a message to many.
Messages and reminders are for free.
Messages can only be not sent to the status "not sent".
First the invitation, save the date or card need to be sent to a recipient before he can be sent a message/reminder.