Me and my friend just want something to brainstorm and write our ideas down on. Cryptpad is a decent I've considered, it has rich text editing which I don't need though it's nice.
Joplin has a sharing feature that allows multiple user editing of an entire notebook.
The downside is that both individuals need to have Joplin installed and either have Joplin cloud accounts or a self-hosted installation of Joplin server, which in either case may be more effort than is worth it for you.