I guess you are talking when you tag someone in photo. If thats the case, no you don't need to have an account of the person you are tagging. But if that person doesn't have account or if there is a typo mistake in tagging someone even if he or she has an account then it won't be clickable (Hyperlinkable).
Regarding adding some one in profile, you can add anyone irrespective of its having account or not. If it doesn't have it won't be hyper linkable, and if he has it would be so. If he has an account and you wish that he Should NOT be hyper linkable then you could make a typo mistake explicitly.
Hope this helps.!
Let me know if anything is required
Thanks
