We keep things pretty simple and have a rack that is in our (admittedly large) loading dock/entrance area and volunteers/visitors just lock their bikes up. It keeps them separate from donations and secure. Similarly, we have lockers that volunteers are encouraged to use to keep their possessions safe from unscrupulous folks or from thinking something is a donation.
We also have a "project room" for bikes that are being worked on, and each bike that doesn't belong to the co-op gets a tag with the owners name, number, project start date, and project 'log' to make sure people are actively working on bikes, along with a basic release stating that OCBC isn't responsible in the case of damage etc that the shop user signs. We also technically require that projects be removed within 2 months and be worked on at least every 2 weeks, though have been lax about enforcing this.