The groups or labels created in the Contacts app on the OnePlus smartphone would be useless if you don't add members or contacts.

Adding new members can be done at any time by single selection or multiple selection, by tapping and holding the first contact to select it and others.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Contacts app
Step 2:  Open the  menu
Step 3:  Choose a  label/group
Step 4:  Tap on  Add contacts
Step 5:  Tap and hold a  contact
Step 6:  Select more  contacts  if desired
Step 7:  Tap on  Save
  1. Open the Contacts app
  2. Open the menu
  3. Choose a label/group
  4. Tap on Add contacts
  5. Tap and hold a contact
  6. Select more contacts if desired
  7. Tap on Save

OnePlus Instructions