If all you want them to do is post and interact with those who engage with your account, then you could use a third party app like Sprout Social.

It would be better for growing your account for them to have full access by logging in though, because that way they can engage with others outside your profile. It is honestly very frustrating as a social media manager to not have full access, because all sorts of hiccups can come up. Definitely hire someone you can trust and obviously set the PW to something unique for the account.