Hi Jon B,
This can happen when both people are using Gmail‑based Teams accounts. Even if you’re both on Teams, those accounts don’t always appear in search, so you may not be able to find each other by name or add each other directly to Contacts.
One thing to try is starting a new chat by entering the full email address (rather than the name) and pressing Enter. If Teams can resolve the account, it will usually create the chat from there.
If that doesn’t work, the most reliable option is to invite the other person as a guest to a Team or a meeting. Once they accept the invitation, you should be able to chat with them normally.
It’s also helpful to confirm that you’re both using the same type of Teams (personal vs work), as mixed account types can affect discoverability.
Please let me know if this proves useful to you, or if you would like further assistance.
I'm looking forward to your reply.