Here is what is needed specifically:

1. Settings > Membership Levels

Add a field under settings that contains the coupon code. The code is tied to the Standard Level Membership.

2. m/membership/index - In the Available Membership Types section First Row

Icon | Text | Text Field | Submit

Icon - A default image can be placed here.
Text - (Launguage Settings) customized per user. Ours would be something like:"ABCD Association members enter your member code:"

Upon Submit see more it matches the code entered in #2 with the code in #1. If True then membership equals Active. If False, then error message (language file editable).