Depending on the scope of your project, users may be submitting data, photographs, audio or other types of content. It should be clear to contributors what permissions they are granting for the project to use the information they provide - this will build trust around how the data and content they provide will be used.
Without clear rights to reuse data, the project may face issues around licensing the combined dataset for reuse by others.
A good contributor licence will cover the full range of contributions that users can make. For example over both the data and any related content (eg photographs, audio, comments).
The licence should only ask for the rights that the project needs, eg to support onward sharing of the data and to manage the service.
The licence should reuse existing open licences, for example by asking contributors to say whether the data and photographs are submitted under a Creative Commons licence.
An alternative to asking for a blanket licence to cover all content submissions is to allow users to choose a standard licence at the point of contribution. This would then be attached to the relevant content when it is shared. This works for individual items of content, but for data contributions, a single blanket licence is preferable as it ensures the entire dataset can be reused under a consistent set of rights.
Changing licence part way through a project is difficult. For example, if additional rights are provided, then existing users will need to agree to the change. This coordination can take time and effort. Ideally the contributor licence will be agreed at the start of the project, in conjunction with potential users.