In order for you to show only groups in chat.
Step 1:
Login to CometChat admin panel.
Step 2:
From Left Menu Click on "Settings->General"
Step 3:
Search for "Recent Chat" And change value of "Disable Recent Chats?" to "Yes"
Step 4:
Search for "One-on-one Chat" And change value of "Disable Contacts Chats?" to "Yes"
Step 5:
Now In chat, you can see Only Group.