Adding new user profile information

I hope this is the right sub-forum for this…

I need a simple way to add ONE new field of information to the user profile page. To be specific it would be a user’s membership ID number.

I have searched and searched and every page I have found with modifications tips seem to use the same simple example of adding a Twitter field (something which is ALREADY in the core, but just not enabled). There are also plugins, but these add new information fields via the registration process. In other words, the user enters in the information.

What I specifically need is a new field: Membership ID which should only be editable by an Administrator (or I suppose Editor could as well). The user should not be able to change their own ID number.

Can someone please point me to a plugin or tutorial for doing this? Like I said, my Googling gets me tons of results on adding editable fields or registration fields, but nothing on Admin-only fields.

Thank you.
Ray