Form and subform for partial data entry

While I use Access often, I'm not much of a programmer but can read most of
it.

My question is I am trying to build a form which pulls information from two
different places:

ContactTable
GroupTable

All of the information from the ContactTable is found within the GroupTable.
My objective is to have the user open the form and see the contact
information in a subform triggered from a vendor number, but not be able to
change that information while being able to create a new record from other
fields.

Also, if the vendor number has not been used before, or no contact
information is available unlock the subform so the user can enter address
information.

Wish I could explain this better.