For this you need to Add your card and create a Billing Account:
What is a Billing account:
Here you add your card details, but more importantly you add the people you want to pay for. you can also add people from outside your own team/collaborators if you want to pay for them.
1. Goto your Tandem Console
1.1 If you click on labels, you can see the label created by you and also the collaborators you have added.
2. Click on 'Upgrade' button
2.1 You see the collaborators of your label.
2.2 You see the 'Add card' button
Click on Add Card to add the card details as mentioned in the following article.
3. Billing Account:
Once you have added the card details , you can do the following:
3.1 You see the collaborators of your label and decide if you want to pay for them by clicking on 'Add Billing'
Now you can see this user as paid and you can also remove by clicking on 'X'
3.2 You also see 'Add collaborators to your Billing Account'. Here you can add people with who you are not sharing labels, but still want to make payments for them , to the Billing Account.
4. Now you see all 3 people ( including yourself) added to the billing account.