free - no registration - without mobile number/e-mail
You can create a webapp and use it for your group. A chat, a calendar and any number of other pages are then available in the web app. New users can also see all previous information and you don't have to register anywhere or reveal your cell phone number. You can activate a push notification for the chat under Settings.
First step
After accessing powapp.de you can upload an image as an icon and specify the name of the new web app:
Invite
You can then invite someone in the menu at the top right, either directly via QR code or with a link:
Notification
Push notifications can be activated in the menu:
Calendar
In the calendar you can simply tap on a day and create a new entry.
Others can then see and participate in this entry by tapping on the entry and specifying their own time period:
As soon as several people take part in an entry, you can see who is taking part: