Accounts must be "personal" to a named individual. Accounts for Groups or Teams are not permitted.
All individuals must have a Mattermost identity.
Take this link, or go to NextCloud (cloud.extinctionrebellion.nl) and click on "Register" link
Enter your email address - and click "Request Verification Link"
Go to your email, and find the email from NextCloud. The email subject should be like "Verify your Extinction Rebellion Netherlands registration request", and will be from email address "cloud@extinctionrebellion.nl"
If you cannot see such an email in your In-Tray:-
First look also in your Spam folder, and if not there…
For some email systems (like Hotmail) which rely on a list of "Trusted senders" you may need to add this address to trusted senders:- cloud@extinctionrebellion.nl
The email should include text "To create a new account on Extinction Rebellion Netherlands, just click the following link:" followed by a single use URL to click
When you take the link you will be prompted to create a Userid and password.
Please use your Mattermost ID, but without the leading "@" sign, so if you are @MagneticWool on Mattermost, use MagneticWool as a Userid on NextCloud
Your password must contain all of:- upper case letters, lower case letters, digits (1…9) and special characters - eg Tob439H%
The account you can set up like this gives you Read-only access to the various "_Shared" areas of NextCloud.
If you want to have Read-Write access - so able to create and update documents:-
Send a message which includes your NextCloud userid to the Group Admin for your particular Local Group, National Circle, Action etc - and ask them to add you into the Group.