Even after a group has been created, members can still be added to the group, either directly via a contact, where only one can be added at a time, or via the group's edit mode.

Android 9 | LG UX 9.0
Step 1:  Open the  Contacts -App
Step 2:  Tap on  Groups
Step 3:  Tap on desired  Group
Step 4:  Open the  Menu
Step 5:  Tap on  Edit group
Step 6:  Tap on  Add members
Step 7:  Select all desired  contacts
Step 8:  Tap on  Add
Step 9:  Tap on  Save
  1. Open the Contacts-App
  2. Tap on Groups
  3. Tap on desired Group
  4. Open the Menu
  5. Tap on Edit group
  6. Tap on Add members
  7. Select all desired contacts
  8. Tap on Add
  9. Tap on Save

LG Instructions