As soon as you create a neighborhood you can start inviting all your friends that have met the following requirements:
– Have reached level 18 or above
– Have repaired the neighborhood house
A few other things that are worth noting: players also have the choice to reject invitations (if it was done by mistake and a player would like another invitation from you, you can simply restart the game and try sending the invitation again). Also, it is not necessary for you to create a neighborhood, but you can just be apart of one and still invite others to join if you are a leader or a co-leader.