Hi Beth,
Congrats for taking on this task! I'll direct you to a section of our File Exchange called Volunteer Recruitment. A little different than a membership drive, but I'm hoping your could tailor or tweak some of the documents for your purposes. You can download and customize what you need.
I took on membership this year, but don't have a creative bone in my body. Does anyone have a membership poster I can adapt to for my high school drive? I would really appreciate any help!