Join in VS Join in with

Question

... horsewoman, Marlee and her husband also now run the cattle station as a guesthouse.Guests can join in with the farming activities and also go fishing. The Bullo River, which feeds into the Victoria, is famous for its barramundi, ... (The Guardian-Travel)

https://www.theguardian.com/travel/2009/jan/25/travel-women-safari-shopping-wildlife

... Yes.I join in with my college's international activities – we sometimes have Chinese nights. My friends and I have cookouts in our kitchens too, where we make ... (The Guardian)

https://www.theguardian.com/education/2014/oct/15/-sp-meet-a-student-from-hong-kong

... disturb the universe?" Let's not accept these demeaning definitions and limitations.Let's not join in with conversations about superficialities. Let's have more of the truly important conversations. ... (Huffington Post)

https://www.huffingtonpost.com/entry/important-conversations-b_b_9576512

I do not understand why 'with' is necessary in the above examples because 'join in sth' already means participating in something. Is the word necessary, or is another meaning achieved by adding 'with'?

I understand why we add 'with' in other occasions, such as ''Feel free to comment and join in with more ideas.'' and ''I join in with the rest of the crowd.'', but I think these two usages cannot explain the above.

Answer 1

It seems to depend on what has gone before in terms of information.

The hotel offers festivities and you may join in (here, 'with something' is not needed as you already know about the activities)

Guests at the hotel may join in with the festivities (here, 'with something' is needed to explain about the existence of activities).

Rewriting your examples to reflect this concept:

'There are farming and fishing activities and guests can join in.

'my college has international activities and I join in.

'there are conversations about superficiality but let's not join in.