Alter default privileges for a certain column
"alter default privileges" applies to newly (in the future) created tables, not newly created users/roles.  It generally doesn't make much sense to grant a column specific privilege on a table which does not yet exist, as it might not even have that column once the table is created.  

If new users are granted the roleGroup role, then they will automatically have the desired permission on the existing column of the existing table.

I don't think there is a way to automatically grant a role to newly created users, you will have to remember to do it yourself, or script it.