In order to function well as a group, it is important that knowledge is shared when it might be useful to colleagues or future group members. For this purpose, we currently use Slite, which is the platform you are using to read this document. It has an extremely intuitive user-interface and has a free version for academics (email support@slite.com from your @princeton.edu email account to request it).
By default, all channels in Slite are internal to the group with the exception of Group Handbook, which is visible to the public. Additionally, there are private channels in Slite that you can use (if you wish) without having to share all the content with the group. Naturally, keep everything within the "Rosen Group" Slite account work-related even if it is in a private channel.
Personal Knowledge and Task Management
In your day-to-day research, you'll find it very beneficial to have a mechanism for summarizing your ideas and results. For this purpose, I personally recommend Notion (there is a free "Personal Pro" account with your @princeton.edu email), but any note-taking tool will do. You can, of course, use your private Slite channel for this purpose as well if you like.
Similarly, many people benefit from having a system for managing their daily tasks. Any to-do list app is likely suitable. If you are using Notion for your personal knowledge management, you may also find it useful for managing your to-do list. If you are looking for a more minimalist approach, you may like Todoist.
Group Chats
For more casual conversations and discussions, we currently use Slack. Of course, if anything important emerges from discussions there, please consider adding it to the handbook!