Setting permission for Access to SharePoint Document Folders for user not a member of the SharePoint

Hi KimHai,

so, to sum up:

You have 

• A SharePoint site that is shared with a given department
• In that SharePoint Site you have an additional Document Library that you use for your "Employee Engagement platform". Here you store files for your meetings
• You have a Webpage/Sitepage that displays some information

You want to 

• Share the document library and the SitePage with other people who are not members of the department

In that case you have two Options:
1) The bad Option:

• Create a new SharePoint Group. (Gear->Site Information->View All Site Settings->Site Permissions->Create Group). Do not give the group any permissions here. (I think your problem lies here as you have given the group permissions to whole site)
• Go to your Document library and select Gear->Library-Settings->More library setting -> Permissions for this document library
• Select "Stop inheriting Permissions" to break the permission inheritance to this library.
• Select "Grant Permissions" and grant your group the "Contribute" Permission on that library.

Now members of the external group have access to all the files in the library, but not to the rest of the site.

Now go into your SitePages library (Gear->Site Contents->Site Pages) and find the Sitepage you want to share.

• Click on the three dots and select "Manage Access". In the dialog select "Advanced"
• Select "Stop inheriting Permissions" to break the permission inheritance to this SitePage.
• Select "Grant Permissions" and grant your group the "Contribute" Permission (Or View depending on your requirements) on that SitePage.

Now the members of the external group have access to both the document library and the site page.

But you will have issues:

• The external Users won't have any navigation to get there (Navigation is only visible for members of the site).  So you will need to provide them with direct links to both.
• They won't be able to see images you embedded on the page (If the images are stored i.e. in the Site Assets library and you have not given the external users permissions to that library ). 

2) The better Option:

The problem is, that you are mixing more than one requirement into a single site. And that might lead to problems down the road.

My suggestion is:

• Create a new SharePoint Site "Employee Engagement platform"
• Move the Document Library and the Site Page there
• Give your department and the external users permissions to the site.

As a result you will have two sites but both just use the default permission set.

They both can grow without impairing the other.