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 "email@example.com"
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:- firstname.lastname@example.org