For smartphone and PC:
Open the user display at the top left corner of the screen, next to the number of people in the session. Then, select the email icon next to the person you want to chat with to open an individual text chat box. The message will be sent only to that person.

For MetaQuest:
Press the menu button on the left controller to open the menu screen. Select the number of people in the session in the top left of the menu screen to display users. Then, select the email icon next to the person you want to chat with to open an individual text chat box. The message will be sent only to that person.