Everything belongs to someone

The footage the group has created belongs to the group and others cannot legally use this in the same way it may not be legal for the group to use something others have filmed.

Always check the copyright of everything. Various stock footage sites might stipulate that the person who uploaded a picture doesn't require crediting when it is used, and this is often something that can be included in a search on a site, but it is still important to check. If the owner of a piece of music has stipulated that their music can be used if you include their name in your end credits, then you must observe this.

Creative Commons

Creative Commons has a wealth of useful guidance on what to look for and what to be aware of when searching for footage and image assets that have been released for use under the Creative Commons license. It is important to be certain of what you have permission to use other you may find your video is taken down very quickly, at the very least.