There is a range of materials that you might need to support involvement. Strictly speaking, this is part of delivering your plan, but it often happens during the planning phase.
For a service-user reference group, you will need terms of reference, role descriptions for members, policies and procedures, information about your organisation, and the work the users will influence. You will find some examples in: Delivering involvement: establishing a user group.
For a one-off engagement workshop, you will need information about the organisation, the issues or work under discussion, flyers, posters and promotional materials, etc. You will find some examples in: Delivering involvement: engagement events.
If you think through the materials you need early in the process, you will have time to develop what you need.