Members can remove from the contact group either via the contact itself or via the group's edit mode, the latter being faster and easier if you want to remove multiple contacts at once.

Android 10 | ColorOS 7.0
Step 1:  Open the  Contacts -App
Step 2:  Open the  Menu
Step 3:  Tap on  My Groups
Step 4:  Tap on  Group  you want
Step 5:  Open the  Menu
Step 6:  Tap on  Edit
Step 7:  Select all  Contacts  you want
Step 8:  Tap on  Remove
Step 9:  Confirm with  Remove Member
  1. Open the Contacts-App
  2. Open the Menu
  3. Tap on My Groups
  4. Tap on Group you want
  5. Open the Menu
  6. Tap on Edit
  7. Select all Contacts you want
  8. Tap on Remove
  9. Confirm with Remove Member

Oppo Instructions