An adult person who wishes to be adopted may do so, and an adult adoption is generally simpler and quicker than the adoption of a minor child. This is because an adult can consent to his/her own adoption, and does not have to get the biological parent’s permission.
An adult adoption may include a name change for the adoptee, but it doesn’t have to.
The final and legal effect of an adult adoption is to sever the legal parental ties to the previous, still living, biological parent(s), and create new ties with the adoptive parent(s). Therefore, the adoptee must realize that s/he will no longer be the legal child of the former parent whose parental ties have been severed. (this can come into play if the former biological parent becomes ill or dies after the adoption. The adoptee will no longer have rights as “family” as to that former parent and may not be able to get in to the hospital, have a voice in decisions, or inherit).
The adult who is seeking adoption could also ask for a court-ordered name change, and change his last name to that of the family that he feels close to. The name change will give him the same last name as the family, but will not sever the prior relationship with his biological parents, nor will it create the parent-child relationship with the “new” family.
Both adult adoption and adult name change are relatively uncomplicated procedures legally (unless the person seeking the name change has a criminal history or it appears that s/he is trying to run from obligations or defraud creditors by the name change).
If the purpose of the legal action is to make the potential adoptee a full-fledged legal member of the family, with all the rights that family members have towards each other, including inheritance, then an adult adoption is the way to go.
If, on the other hand, the “adoptee” does not want to sever the legal ties with his/her biological family, but wants to bear the last name of his/her “other family”, a name change may be just the right remedy.