I have some friends that play on epic. I am using the whitelist and have whitelisted people on steam but when I attempt to whitelist an epic account is says it's an invalid steam id. Does whitelist not work with epic?

If you have mods on your server then yes epic won't connect. Epic accounts aren't steam also.

A lot of features, including whitelisting, don't easily work with Epic due to how ARK:SE was built from the ground up with just Steam in mind. 

I figured out how to whitelist epic accounts but it's super annoying. I have to disable the whitelist. allow the epic gamer to join...from there I can click his link from the player list and get a code for him to add to the whitelist then I can reenable the whitelist. super annoying but it works.