If the certified mail is standard certified mail then anyone 18 or older at the residence can sign for and accept the certified mail.
But if the certified mail is restricted certified mail then only the person who's named on the certified mail can sign for the mail and must present ID to the postal employees before the restricted certified mail is delivered to the recipient.
But if you cannot be present at the address to sign for regular certified mail then anyone as long as they are 18 or older can sign for the mail.
Even a neighbor can sign for the certified mail for you if you tell the post office that they can deliver it to that address and have then sign for it.
I had some certified mail coming to my address and had to leave on an emergency trip to be out of town and had a neighbor I trusted sign the certified mail for me and accept it.