Airtable credits (received by, for example, referring others to Airtable or installing the Airtable mobile app) can be applied toward a team which is on a paid plan.
There are two ways to apply your credits toward a team (note that you must be an owner of a team in order to apply credits toward a team).
The first way is by going to your account page. To access your account page, click the circular face icon in the top right of the home screen, then click "Account" from the dropdown menu. (Alternatively, just go straight to https://airtable.com/account.)
On the "Your Account" page, click the blue "applying it to a team" link.
Then, select the team to which you'd like to apply your credits.
The second way of applying your credits to a team is to go straight to that team's account page. To get to a team's account page, click the dropdown arrow next to the name of the team, then click "Account" from the menu.
You'll then be brought to the team account page. In the plan & billing section, there is an option to transfer personal credits to the team.
Clicking the blue "transfer" will bring up a window in which you can enter the number of credits you wish to transfer.