Stash does offer $5 cash for signing up and depositing $5 in your account. You can take the $5 and buy free stocks with it. Since $5 is not much, you will likely be buying fractional shares.
You can receive free cash of $5 for signing up and making your first investment. You can take the $5 and buy free stocks with it. Acorns also has a rewards program where you can earn $5 for inviting friends.