What do you mean about the label of the people picker?
If you want to change the column name, we can go to list settings to change it.
If you want to change the help text of the people picker, we can use the code below to achieve it.
You have a classic case of an orphaned SharePoint user. SharePoint stores all users as list items in a SharePoint list.
If you just want to remove a single orphaned account - open this url
and remove the user manually.
If you want to do a proper cleaning - you will probably need a powerShell script. ...
Absolutly, showing the users Group property is pretty simple if you are familiar with item display templates. Adding the additional user property "Group" is also pretty simple but in most cases the hard part is 1) getting the users to update this property or sync it from AD property and 2) ensure the data quality of this property as group names changes over ...