Pledge by

That depends on what you mean by pledge. If you mean a promise by your members (other than a financial contribution), it's fine, although i would add the definite or the indefinite article to the beginning: The pledge by members of our association. --or-- A pledge by members of our association.

If, however, by pledge you mean a promise to make a financial contribution, then i would use the plural form: Pledges by members of our association.

Many thanks, Setanta.