The only possibility is to make this customer a company administrator, so he can manage his own users.
There is, unfortunately, no other alternatives and no function such as sub-users.
Hint: However, we do not recommend to do this. This is only on the technical side the solution for it. We highly recommend having only trusted company administrators of your own company because company administrators have all managing rights over your license.
I hope this could help. If not, do not hesitate to ask your questions here.
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