To grant a user access to a LeadGroup, an administrative account will need to access the LeadGroup section of the “Workflow” tab.
This will bring you to a page where all LeadGroups in your company are listed. If you look carefully to the left of each LeadGroup, you will notice a small box. In order to grant access to that LeadGroup, you must check that box.
Once you have checked the box(es) of the LeadGroup(s) you wish to grant the user access to, scroll down till you see the "User List". Highlight the name of the user you have just added and then hit the "Submit" button. Please note that this is the only way to assign someone access to a LeadGroup outside of making them an Administrator. Setting them as the default owner does not grant access to LeadGroup.
The user will now be able to see all leads in that LeadGroup. You can verify the user has access by clicking on the "Information Bubble" to the left of the LeadGroup. (It is the little bubble with the "i" in the middle of it.) When you click on it, it will show you the LeadGroup's information, including who has access to it.