Hi, welcome to the Atlassian Community!
With Trello Premium, billing is based on the number of Workspace members, which includes the core team within your company's Workspace. In your case, as you have 6 internal members, your company Workspace would be billed for those 6 Premium members.
Regarding guests, there's an important distinction to keep in mind: single-board guests are not counted towards your billing, but if a guest is invited to multiple boards in your Premium Workspace, they are considered multi-board guests and are billable.
If your guest already has a personal Trello account for which they're paying, inviting them to your Workspace as a multi-board guest would not affect their personal billing. They would continue to pay for their personal account separately. However, in your Workspace, this guest would count as an additional member for billing purposes.
So, if you invite a guest to more than one board in your Premium Workspace, your company would be billed for 7 members in total (6 internal members plus 1 multi-board guest). It's also worth noting that you can add or remove multi-board guests at any time, and your billing will be adjusted accordingly.
Hopefully this helps, let me know if you have any questions!
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