Wargaming official tournaments all utilize the ATS (Automated Tournament System). The purpose of this system is to make joining the tournaments easy and automated. Thanks to this system, the hamsters powering our servers take the stress of setting up battles away from the eSports team.
A few minutes before the first battle of the match, all team members will receive invitations to a special battle. This invitation will be displayed as a popup message similar to the message visible on the screenshot below. To join the battle click on "accept".
If you know you are expecting the message (i.e. other team members have got it already) or you’ve accidentally closed it, please don’t panic. First click the button under the “Battle” button and select “Special Battles” from the dropdown menu. This should bring the window back to the middle of your screen.
A tournament room is similar to a company battle room. Every invited team member can join the battle roster, so team members don't need to wait for the captain to assign everyone to the team. To avoid a technical defeat, please make sure to select a vehicle of the correct tier.
Don't forget to set yourself to "Ready!" before the battle starts, and move yourself to the right side by double-clicking on your name or clicking on the right arrow. Your name then appears in the right column, as depicted below:
When the match is over, the "special rally point" window will be automatically closed.
Disclaimer: Please remember that the above guide is general and may not apply to every tournament you come across. For instance, it only applies to Wargaming tournaments and not to tournaments organised by any third party.