First click on "Chats" in the footer.
In the next step, tap the red plus sign next to "Chat" in the header.
Here you will see all the members in Sdui with whom you can start a chat. You can also search for individuals using the search bar.
Tap the person you'd like to chat with and the chat window will open.
You can now open the chat with this person directly from your chat overview.
If you want to start a chat with a new group, click on "Create group with chat" and follow the steps under "How do I create a new group?".