Correct me if I'm wrong but in order to spend from that address, two signatures are needed. One mine and one node's. That's why it is larger than the others. (?)
Correct.
How can Charlie join this? Does he need to open his own channel? Or join mine? From my second wallet I generate a lightning address and I paste it on my first's "Pay to". Once I do, it probably decodes it to its HTLC form (256-bit in hex). [...] And that's reasonable. They probably have to connect somehow with "Bob" or in this case "STRANGESET".
You cannot join an existing channel. Right now, payment channels are established only between two people (
channel factories are kind of close to what you mentioned, but they are not a thing yet). "Charlie" needs to open a channel to either the same node or any other that is somehow connected to your node. Note that you won't be able to receive any funds through that channel unless you spend some coins. If "Charlie" does not open a channel with the same node as you then his payment will need to be routed through other channels and it might fail due to lack of routes or enough liquidity in the intermediary channels.