Ticket purchases are for one passenger at a time.
To buy more than one ticket simply repeat your purchase (hit "Buy ticket" from the ticket listing page) and configure the details of the other passenger.
Alternatively, you can also now hit the "Buy another" button on the order confirmation page:
FAQ
Do I need to register another account per passenger?
No. You only need one account as a customer, and all of your tickets, for multiple passengers, can be accessed from your single account.
How can I buy more than one ticket with Direct Debit?
Follow the same steps above. You will not need to create a Direct Debit mandate again, this only ever needs to be done once per customer. The next ticket purchase you create will raise payments/subscription against your existing Direct Debit mandate that you previously created. You will receive advance warning by email of any payments, and you will receive confirmation of each ticket order by email.
It says there are only a certain number of tickets left but I need more than that. Can I buy more tickets?
If there is a warning that there is only a certain amount of tickets left, but you need more than this, DO NOT make any purchases until you've spoken with your transport operator. This likely means the service is reaching capacity but it is still worth speaking with your transport operator to find out if they can increase capacity or even put you on a waiting list in the event another customer leaves.