@StuartBell8
Hi there - If I understand correctly, you are trying to set up a meeting with someone internal to your organization and someone external to your organization. It is not clear if you want to set up a team to interact and collaborate or if you simply want to join a meeting together and talk / video / PPT on a call. If the latter, you don't have to be in the same organization at all. You can set up a meeting in Outlook at make it a Teams meeting, or, you can set up a meeting in Teams and put your attendees on the 'to' line. At the time of the meeting, everyone will join the call using the Teams desktop client OR the web client OR the mobile client - whichever they choose to join with.
if you are trying to set up a team and want to include someone external so you can share documents, collaborate regularly, etc., you can do that, too. The person external to your organization would be a 'guest' to the team and have all permissions allowed to guests in your organization. The difference in this case is that when they want to collaborate with you, they will need to switch from their own tenant to your tenant. We are working on improving this painful experience right now. In the meantime, if it is required to be in 2 tenants at the same time, many people open one instance up in the web client and some even have multiple tabs.
I hope that answers your question.
