In a contact group, we can add members both during creation and afterwards.

Android 10 (One UI 2.0)
Step 1:  Open the  Contacts -App
Step 2:  Open the  menu
Step 3:  Tap on  Groups
Step 4:  Tap on desired  Group
Step 5:  Tap on  Edit
Step 6:  Tap on  Add member
Step 7:  Select all desired  Contacts
Step 8:  Tap on  Done
Step 9:  Confirm with  Save
  1. Open the Contacts-App
  2. Open the menu
  3. Tap on Groups
  4. Tap on desired Group
  5. Tap on Edit
  6. Tap on Add member
  7. Select all desired Contacts
  8. Tap on Done
  9. Confirm with Save

Samsung Instructions