Important: To purchase additional seats you must be the Business Membership "Owner". Business Membership "Managers" cannot purchase additional seats, they can add/remove/invite members only, and only if there are membership seats available.
1) Go to your team that you would like to add more members to
2) Select "Team Settings", "Change Seats"
3) Change seat count to required amount (i.e. if you need 5 seats for your business membership, enter 5) and click submit.
4) Finish payment checkout. note: your additional seat pricing will be prorated based on your initial business membership purchase date