The way you have been doing things is correct. We have temps work for us from time to time as well and, if they are working on-site in our office, they get a company-issued computer. If they want to BYOD, they only get access to our public Wi-Fi which is internet-access-only and segregated from the rest of our network.
We are a marketing company and we regularly hire contract art directors and designers. We currently set them up in AD with the correct security permissions, give them an email address and set up a company owned MacBook Pro for them to use during their contract. My CFO is now asking if we can let them use their own personal equipment while they are here. I think this is a potential security risk but wanted to see how other companies handle temporary employees. Do you set up company equipment for contract employees?