Easiest way is your client sets up their Make organization, invites you to it.
Then, you provide instructions for them on how to setup a connection for each of their social media accounts.
Or, your client installs Buffer which can already post to multiple accounts. You create a new team for each separate client to connect their Buffer accounts in.
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Did you know, the concepts of about 70% of questions asked on this forum are already covered in the Make Academy. Investing some effort into it will save you lots of time and frustration using Make later!