I see the difficulty. There’s a neat trick with webdav + nextcloud to not need collaborators create yet another account, but to delegate them some OS integrated shared storage from your own account:
Use issued share tokens (and optional password) on folders as identity:
For example, with a share link
https://murena.io/s/kFy9Lek5smbogus,kFy9Lek5smboguswill be the username.
It’s important to use the https://murena.io/public.php/webdav url. You even get file locks.
Create a read+write folder (//shared/person2-or-project) and let person2 mount that in their OS, there’s instructions and screenshots for each OS further up that doc article. You need to adjust the url though.