We are converting from Skype for Business and WebEx Teams to MS Teams. We are federated with an external company. I cannot find users from the external federated organization using the Search bar at the top of the Chat screen or in when searching to add a contact to a group. If merely says " We didn't find any matches". It doesn't give me the prompt to search externally.
However, the user from the external organization can find me in their search, and if they ping me first, then we can chat. From that point on, the search at the top of the main chat window will find them in past chats, but not by their email address. I cannot add them to a group even after they have pinged me.