correct usage of "to join and to join in"

I have trouble using "join" and "join in" correctly.

(1) Would you like to join us for Jack's birthday party?
(2) We want to celebrate Jack's birthday next month. Would you like to join in the party?

(3) I would like to join you and your friends for lunch tomorrow.
(4) I would like to join in the special lunch you are organizing for your uncle.

(5) I want to join the video game event next week.
(6) I want to join in the video game event next week.

(7) I want to join the yoga festival next week.
(8) I want to join in the yoga festival next week.

I learn a lot of English from my neighbors when I talk to them regularly. I am not sure if they are native speakers. They think (1) to (4) are grammatically correct because the rule is that you join someone for something and join in something. However, both join and join in work for (5) to (8) when you are talking about events or festivals. When you say "join an event or a festival", you are a member. When you say "join in an event or a festival", you are participating in it.

Please give me your opinion. Thanks in advance.