"Who've" is a contraction of the words "who" and "have." There are a few synonyms for "who've" depending on the context it is being used. For instance, if you are looking for alternative ways to express the possessive pronoun "who has," you might use "whose" or "whom has." Alternatively, if you want to emphasize collective ownership, you can use "whose everyone has" or "whose each individual has." When using "who've" in a question regarding people's actions, you may replace it with "who have" or "who had." Finally, "who've" can be substituted with "who possess," "those who own," or "individuals who hold".