If so was it very complicated as far as paperwork and other legal stuff?
I've edited a few anthologies for myself and other publishers.
It can be as complicated or not as you depend. Most of the extra stuff in contracts done by publishers is to cover yourself in the case of something not working out.
The simplest is to write a contract that specifies:
1) what control you might have over editing or whoever is compiling it.
2) what each author is paid and when.
3) rights: i.e. if the material in the compilation is exclusive, or if the author can also use the material in their own compilation or sell separately.
4) how the author can get out of the contract. Whether it expires in a certain timeframe, and/or if they can give notice and how long before the material reverts after notice.
If it is a single piece of fiction that involves multiple authors working on it, then obviously the rights are a different issue since it will always be a combined work. I've also done a lot of this, and I do still suggest a contract, but it is a completely different entity.