This has got to be easier than I'm making it out to be.  I have 3 SharePoint groups listed under groups on a site Vistitors group, Member group and Owner Group.  I want to add forth group that is already in SharePoint named "Portal User" to that site.  The only thing can find is to add a new group.  How do I add existing SharePoint group to an site

Thanks 

SharePoint groups are defined at the Site Collection level, and if you break inheritance, then at the site level.

Where is your "Portal User" group defined? In another site collection?

 

Best practice in this case might be to create an Active Directory group for the Portal Users people and then add that group to each site collection as needed. This moves the group management out of SharePoint and makes it available across all sites, site collections and even other systems.

 

 

From the site that you want to add the group to; go to Site Actions, Site Settings, and under Users and Permissions select People and groups.  From here you want to select the group to add from the menu on the left, if it is not listed select "more".  Once the group is selected you should see the members of the group listed.  Now select settings drop down menu and Group Settings. The section titled "Give Group Permission to this Site" is where you will add the group to the site that you are working with.  Fill this section out as necessary and select OK.  Your SharePoint Group is now added.

• Proposed as answer by N_Smoot Monday, October 28, 2013 6:51 PM

In case , like me, you ended up here with this question and the answers didn't help. Here's what helped me....

You have to go to the Site in Question and click...

Site Action > Site Permissions

Then in the Ribbon click "Grant Permissions"

In the Select Users section (Top of Overlay), type the name of the EXISTING SharePoint Group that you want to add to this site, search by clicking the little person icon with the check (or first click the Address Book Icon and search for the group that way) 

IMPORTANT STEP!!

Then under the Grant Permissions section on the Over Lay (Pop up) Select he "Grant user permissions directly" radio button!!!!

Give the desired level of permissions for this particular site.

 Cheers!

brilliant!

Site Action > Site Permissions

Then in the Ribbon click "Grant Permissions"

In the Select Users section (Top of Overlay), type the name of the EXISTING SharePoint Group that you want to add to this site, search by clicking the little person icon with the check (or first click the Address Book Icon and search for the group that way)