From your query I am considering the fact that most of your users are having the same permission. If that is the case and you want to have some User Defined Fields (UDF) available to a specific group and not to others, then the best way to do it is as follows.
Create a new permission based on their existing permission. (This will ensure that there is no change in permissions)
In the new permission enable the required UDF's. (Whilst the original permission remains unchanged)
Disable the original permission in the selected user(s) profile. (You can use Mass edit if there are more than one user to be edited)
Enable the new permission the selected user(s) profile.
Now you will have 2 sets of users with the same permission, but different UDF access.