In Confluence 5.8.6 (and 5.8.5 on another installation) it is not possible to edit (or remove) a groups permission to a particular space once it is added. When trying to modify, or remove the permission, the javascript console generates the following error message:

Uncaught Error: Syntax error, unrecognized expression: [name=confluence_initial_viewspace_group_test group]

To reproduce:
(0. If necessary, create a group containg a blank space, i.e. "test group", in Confluence or your external User Directory)
1. Open "Edit space permission" in a space of your choosing and choose "Edit permissions"
2. Under Groups, search for the group with a name containing a blank space. Click "Add"
3. Modify the groups permissions. In my reproduction I try to add "Add" permission for Pages.
4. Select "Save all" at the bottom. The button is grayed out, but nothing happens. The javascript console generates an error message as shown above.

At this point, nothing more happens. Reloading the page shows that the group has been added to the space permission scheme, but is impossible to modify/remove.