The groups created on the Xiaomi smartphone only make sense when you add contacts, which we can do either via the contact details, or via the group settings to select several.

Android 10 (MIUI 12.0)
Step 1:  Open the  Contacts app
Step 2:  Tap on  My groups
Step 3:  Select a  group
Step 4:  Tap on  Add
Step 5:  Choose one or more  contacts
Step 6:  Tap on  OK
  1. Open the Contacts app
  2. Tap on My groups
  3. Select a group
  4. Tap on Add
  5. Choose one or more contacts
  6. Tap on OK

Xiaomi Instructions