Joining a writing group doesn't even mean that you have to write with other people! Most of the time, if you have a writing group, you mainly write in a room and share feedback. In my writing group, one that I have with my best friends, we mainly just post our poems and stories onto our website and share feedback.
Writing novellas or novels with friends can be really fun as well. You and your friend can write each chapter together, or switch off. It all depends on what both of you agree on. Writing with other human being (so not your dog, even though he might be your best friend) allows you to grow as a writer by taking other people's ideas and critisim into account.
If you end up writing with a partner, remember that you can't be in charge the whole time. They have brilliant ideas, just as you have brilliant ideas, too. Somehow, you must forget about being right and combine both views to create a wonderful story.
I am currently writing a Warrior Cats fanfiction with my friend. You can find our story, River's Descending, here, if you have a Wattpad account. We hope you like it!
-- Brooke_Writer and lynxkitten65