If you want freelancers to track their time on projects, you should invite them and give them a login, just like other people in your account. You pay for “active” people with logins, so once the job is done you can archive that freelancer and use their seat for someone else. Some customers have a few rotating seats designated for freelancer use.
If you do not need to account for the time freelancers spend and just want to track their cost to the project, you can add them as an expense against the project budget.
Tip: If you do not want these freelancers to have company-wide information, you should set their permission level to Contractor.