The Center for Cooperative Media has:
A database of journalism collaborations from around the world available in English, Spanish and Portuguese;
Heather Bryant's Project Facet has a collaborative journalism workbook available in English, Spanish and French. It will soon be available in Italian.
The six types of collaborative journalism and the pros and cons of each, from Nieman Lab.
Six lessons from collaborative journalism projects, from Global Investigative Journalism Network.
Ten tips for successful collaborations from 2018 JSK fellows, available in five languages.
Slack : This is a useful tool for communicating quickly with partners, sharing news and uploading files.
Trello: This helps with task and project management, and it is especially helpful if you need to collaborate reporting or distribution among teams.
Airtable: A sophisticated tool used for project planning and tracking. You can also use it to collect callout responses.
DocumentCloud: A place to share and store documents.
Datashare: Allows you to analyze documents and extract text from uploaded files.
MuckRock: Allows you to track, send and share FOIA requests all in one place; they'll send payments for you.
Todoist : A useful tool for individual task management that you can use on both desktop and mobile.