Messages should be sent to anyone that you would like to include in your project. We do suggest though that you make contact with individuals through customary means first (phone call, email, genealogical gathering, etc.). This will help keep your invite from being regarded as spam.
If you use the Join Request option, this should be sent to people that you approve of allowing in the group. If you do not use the option, you can use it to email individuals who may have already spoken to you about the group. This is one way of making sure that they order and are included in your group.