This page shows the different variations of the Wiadomość grupowa from an Icons8 icon pack. Feel free to resize and recolor the icon, and then download the icon in PNG format for free. Icons are available in PNG and vector.
The icon is a picture of group message. The icon is the shape of two rectangles. The rectangle in front of the other one looks like an envelope. The envelope looking rectangle is slightly lower and to the right of the rectangle behind it.