Let the employees know that they're not the only ones contributing by emphasizing the company aspect of helping others. Also, make it easy for them to contribute by offering payroll deduction.
From: Jim Karter
Subject: <subject of memo>
Abc Company is proud of the level of support its employees have shown toward assisting others. Whether on a private volunteer level or through the company giving program, our employees play a vital and contributing factor in helping our community.
Abc Company is supporting <name of charity drive>. By giving <suggested amount of donation>, you <benefits>. You may <method of contribution>.
Let's keep our great record of helping our community. Thanks for your participation.