I want to use the example of @calmh and illustrate it. @calmh please feel free to use any pictures and correct me if i am wrong.
Lets assume that Alice already has connections to Charlie (trusted as introducer and trustes Alice as introducer) and Eve (not trusted as an introducer, doesnt trust Alice as an introducer). They are all sharing only one folder.
This means that Charlie will add Eve, but is not added back:
– Now Bob, an otherwise blank node, is added to Alice. Bob adds Alice to his configuration too and sets the introducer flag.-- < This is the only user interaction in this explanation.
Alice sends her cluster configuration. Bob thus adds Charlie (as an introducer node, since he was trusted by Alice) and Eve (as a normal node).
Similarly, Alice will send information about Bob to Charlie and Eve. Charlie trusts Alice as an introducer and will be add Bob, but Eve does not trust Alice and so will not accept the connection. Note that Alice doesnt accept Bob as an introducer and so Charlie will also not accept Bob as an introducer.
Note that propagation can be a bit delayed currently, since the information is only sent at connection establishment.
Note that this is happening per folder.