Business Account has 5 user licenses initially. You can buy additional number of user licenses at any time.
There is no limit of how many users you can have in your business.

Business account is registered by one person, the business account owner. Owner can invite other users to Business, manage all aspects of business account.
All users who have been invited to Business have full access to all HiTask Business and Premium features.

You don't need to remove your Free account. You just upgrade it!
Go Settings and click on "My account" then "Upgrade to Premium" or "Upgrade to Business".
And all your tasks and projects will be carried over!

Since free accounts have limitation on number of teammates, you'll need to have at least one Premium account for yourself which will "see" all other team members.