When you are in an active call, you can invite another person to join.

1. When on a  call, click the Show Participants    button at the bottom of the screen. A window will pop-up to the right listing the participants in the call.
2. Click the search bar in the Participants window.
3. Start typing the name (or phone number) of the person would like to invite.
4. When you are done typing, press Enter. MS Teams will automatically call the person you have selected.
5. If the person accepts your invitation, the person will join the conference call.