For you to be able to send someone a message that user has to be in your contacts list : go to People -> Youth and choose the person you are looking for. Then press "Add to Contacts". Now, when trying to send an internal message the person will be in your contact list. ( pastor )

No. If you want to send messages you have to be a registred user. ( pastor )

The number of messages is only limited by the space allocated to the site by the hosting server. ( pastor )