If you haven't worked with a freelancer before, but you'd like to save their profile because you plan to invite them to a job in the future, you can use the 'Save profile' button on the left of their profile:
Please note, the 'Save profile' button will only appear when logged in to a client account.
You can access the freelancers you've saved, for example to invite them to apply for a job you've posted, by clicking 'Freelancers' and then 'Saved Freelancers':
Freelancers you've worked with
If you've worked with a freelancer in the past (or present), it's not necessary to save their profile because you will instead find them automatically listed in the "My Freelancers" section of your account: