With Tuple, both participants in a pairing session can draw on the shared screen.
If you're viewing a screen, make sure you're using the "Paint" cursor mode by selecting if from the Guest Toolbar.
If you're sharing your screen, you can configure a hotkey in your preferences to enable Paint mode, or click the icon in the menu bar:
Press the hotkey again, press "Esc", or click the Tuple icon in your menu bar to return to normal.
In either role, just click and drag to paint, and use right-click to clear your paint. In your Preferences, you can choose between waiting until a right-click to clear the paint, or if you want it to clear after a few seconds automatically.