How to give existing user website FTP access?

I’ve been banging my head on this all day. I want to give an existing user website FTP access. Not create a new one. The best I’ve figured out is changing their home directory, but that screws up their email – and I’d really like to only need to give one login (that works for both email & FTP). But I can’t for the life of me figure out how to do this.