How do I have people approving leave in different company accounts?

This can be solved via a one-time workaround implementation.

Let's say your name is Coca and you need to be in Company A, B, C to approve leaves.

Create a Gmail/Hotmail email address for Coca: eg. Coca@gmail.com. Then in this email setting, set auto-forward all email to your own company email account eg Coca@company.com.
Now, go Company A Kakitangan.com account, create employee Coca with this Coca+compA@gmail.com.
a. Make the needed employees report to this Coca account.
b. If this company is not paying salary for Coca, go Payroll Calculator, untick “include in payroll” and save for this Coca account.

Done. You may replicate this practice for other companies.

Gmail has a specific feature if you have more companies later. You only need to create Coca@gmail.com on Gmail once.
Then you can use
Coca+coA@gmail.com
Coca+coB@gmail.com
and different variance so on.
and use it to create multiple Kakitangan.com account, all emails to them will send back to Coca@gmail.com.

Hope that helps.

how to setup Kakitangan.com so you can automate the max for your payroll and people operation https://blog.kakitangan.com/first-time-onboarding-your-company-to-kakitangan-com/