the word 'join' doesn't imply a connection, but the 'link' does. though the 'link' means only a one-way connection. a two-way link is a connection. for example, when when you follow someone on italki you'll make a link. when he (she) approves your request a connection will be established. It's a two-directional linkage. and when the partner answers to you with a message it means the partner joined the conversation. It's simple!

join is different go to dictionary.com