Grammarly Premium for individual users costs $29.95/month when paid month to month, $19.98/month paid quarterly, or $11.66 per month when paid annually.
For teams, Grammarly Business supports 3 to 149 users at $12.50 per user per month. Pricing is opaque (where you have to contact the Grammarly team for pricing) for 150 or more users.
The difference between business and personal premium accounts are aimed at team management, including admin panel, customer support, single sign-on (you should be able to set this up yourself if you already use Onelogin or Okta), and usage stats.
Pro tip: If you have 9 or fewer people, it's more cost-effective to buy one log-in ($11/month, sometimes $6/month) and share it OR have people buy and expense an annual premium account on their own.