If you need to be added to your client's Google My Business account this article will help you out.
When working with your clients, it's usually pretty helpful if you can become a user on their Google My Business account either to be able to connect and post things on their behalf or if you need to help them out with modifying something on their GMB account directly. This link here will explain exactly how you can do it!
https://support.google.com/business/answer/3403100?co=GENIE.Platform%3DDesktop&hl=en&oco=0