Use case example:

Your web site is about listing restaurants in your area. You created a content type (node) to add restaurant information and you've been adding content about the restaurants. These content articles are visible to anybody visiting your site (no access control).
Say a user wants to start a group about French cuisine and want to build up a group of people with a similar interest in French cuisine restaurants in your area. And they also want to add to the group, content related to French cuisine restaurants. But since the content is open for anybody visiting your site, you don't want the content to be specifically belonging to a Group, just have a reference to any content you want to add to the group.

How to:

First of all, do not enable the Group node (gnode) module. This module sets permission control over the content belonging to a group.
Since a Group is an entity, there are other ways to link to other content within a group. In this case, we can use the Entity reference module
Create a Group type, add your fields to the Group type under Manage fields. But here you also add a field of Field type: Entity reference. There are a lot of widgets to set reference to the content type, best to read up more on the Entity reference module.

Views:

If you want to create a View to show the related content, you can do it this way:
Create a new View with Show: Groups of Type:[your group type].
Under Contextual filters, add Group: Group ID and set it up as per normal how to handle it if the filter value is not available (example, Set default: Get Content ID from URL).
Under Relationships, set a reference to the field you've added above. It will be something like this:
Entity Reference: Referenced Entity
A bridge to the Content entity that is referenced via field_group_related_content
Build the rest of your View like you want it with block, pages or content panes for how you want to show it. Use the Related content as reference in your Views Fields to show information of your added content.

Alternative, add all content with a certain Taxonomy term:

If you do something like the Use case example, then you most probably will have the restaurants classified with certain terms, such as a French cuisine term. Then you can automatically add a reference to all content with the same term.
In this case add the same taxonomy vocabulary to your group type, when creating the group select the term you want to use.
In Views, under Relationships, set up a relationship with this vocabulary.
Under Filter criteria, add a filter on your content type, using the same reference you set in Relationships as parent.