Is "join sth" the same as "join in sth"?

The dictionary says they are close in meaning: to get involved in an activity with another person or group.

1. He stared at them but didn't join in the conversation.
2. I don't have time for a drink now, but I'll join you later.

What is the difference?

Question

Is "join sth" the same as "join in sth"?

The dictionary says they are close in meaning: to get involved in an activity with another person or group.

1. He stared at them but didn't join in the conversation. 
2. I don't have time for a drink now, but I'll join you later.

What is the difference?

Answer

If you "join in",  you participate in an activity with a person or some people who are already engaging in that activity.

If you "join" a person or a group, you simply go where they are and/or add yourself to their number. There may or may not be any activity.