Just to confirm the answer to my query, I am quoting a couple of members in another thread:
dashead... you are placed in a neighborhood with other players similar to your "level". And then once a week there is a reshuffling of players in neighborhoods in the attempt to keep players at a certain "level" together.
jaelis...You can trade with anyone in your guild, your neighborhood, or on your friends list. So people in your guild don't have to be in your neighborhood
My understanding therefore is that the message I ocassionally get that a scout has found new members in my neighbourhood, is the games way of saying a redistribution of the neighbourhood members has just occurred so as to keep everyone at a similar level together. Because of this players in the same guild and in my friends list, can end up in different neighbourhoods, hence trade is still permissable.
My experience as detailed in initial comment suggests the corraling of players into a neighbourhood is, in part, defined by the Age they are in and that no-one will be more than two Ages apart either up or down.
Any comment clarifying or confirming this would be welcome.