When a new person signs up, they should be contacted within 24 hours.
It's a simple thing. A quick phone call. The signup and join page on your campaign's site should send a cc'd email to your volunteer coordinator, and they should be making these calls.
A quick call that says:
1) Thank you
2) Asking what the volunteer is most interested in doing
3) When they can come in
I recently signed up to help volunteer on a local race for Congress. The candidate has raised 50k to the incumbent's 2 million. Their only option is to run a grassroots campaign. Yet, 72 hours later, I still haven't received a phone call, email or anything.
Always call volunteer signups within 24 hours and get them working on your campaign.