To access FirstAgenda in your organisation, you need to receive an invitation mail from the organisation which has invited you.
If you have not received a welcome mail, you should contact the FirstAgenda administrator in your organisation - or the person responsible for your team, as they are the only ones who can give you access to FirstAgenda.
Before you contact your administrator, it is a good idea to check that the welcome mail has not been stopped by your mail programmes spam filter or is not to find in the "Junk" folder. This file can be found in the navigation window of your mail programme. Also check the "Clutter" folder if you are using Outlook 2016 or later versions.