I've heard that you have to say "join someone" and "join in something". I am going to make up a few pairs of sentences below.
(1a) I will join you for John's birthday lunch tomorrow.
(1b) I will join in for John's birthday lunch tomorrow.
(2a) I will join you for his retirement party next week.
(2b) I will join in his retirement party next week.
(3a) I will join the international comic event next month.
(3b) I will join in the international comic event next month.
I think both (1) and (2) follow the correct usage. I believe that you can say "join an event" without adding a pronoun after "join" because you will participate in it. "join in an event" is the correct usage.
Am I using "join" and "join in" correctly in all my sentences?