Additional meeting and call-related features and issues within Microsoft Teams for business
Hello Moriah,
In Teams, you can send a message to a visitor user. By choosing conversation and then New conversation to create a new chat, you can invite someone from outside your company to a one-on-one chat. Then, enter the contact's phone number or email address to initiate a chat. Enter a message in the bottom text box to launch the chat, and then click Send.
You can refer to this link as your guide: https://support.microsoft.com/en-us/office/add-...
I hope this helps.
Kind regards,
Jayzar