I refer to Case ID:20320290 and to official document "Add a guest to a team". It says Global admins OR Teams admins and team owners can add a guest to a team in the Teams clients OR in the Teams admin center. However as a Global Admin, I get an error: "We can't find any team members to add." The Microsoft Support has advised me to use uservoice. Right now, to add guest, the global admin has to join the team as an owner then use the Team clients. It is not operationally feasible for the Global Admin to join every team as an owner.