How do I handle multiple temporary contributors?

For a project I am planning to use multiple copy-writers / contributors on an on-and-off basis. Are there services out there I can use for this? Want to avoid going through the hassle of dealing with different tax situations in different countries for each invoice and contract.

It looks like Fiverr for Business and Upwork Teams have some capacity to solve this, but would like to hear from people with actual experience in this.

On a side, it would also be nice to set parameters for the ip part of this (eg. that content can only be republished after x months) through the same service.