The GlideSystem User object has a useful function, which is called getMyGroups. This gives back the sys_ids of current user's group. But the functionality behaves differently depending where it is called from.

If the function is called from Global scope a Java object (com.glide.collections.StringList) is returned:

var currentUserGroups = gs.getUser().getMyGroups();
gs.info(currentUserGroups);
gs.info('Object type: ' + Object.prototype.toString.call(currentUserGroups));

Result:

*** Script: [723aa84f5ba02200502f6ede91f91aea, cfcbad03d711110050f5edcb9e61038f]
*** Script: Object type: [object JavaObject]

When the function is called from Application scope, the type will be a Javascript Array object:

x_149822_va_code_p: 723aa84f5ba02200502f6ede91f91aea,cfcbad03d711110050f5edcb9e61038f
x_149822_va_code_p: Object type: [object Array]

The main problem here is that the StringList class behaves differently like a generic JS Array. For example you cant get an element from the collection based on its index (currentUserGroups[0]).

Javascript compiler exception: Java class "com.glide.collections.StringList" has no public instance field or method named "0". (null.null.script; line 8) in:
var currentUserGroups = gs.getUser().getMyGroups();

The solution below gives a generic way, how this function can be called from both type of Applications:

var currentUserGroups = gs.getUser().getMyGroups();

if (Object.prototype.toString.call(currentUserGroups).match(/^\[object\s(.*)\]$/)[1] == "JavaObject") {
    var arrayUtil = new global.ArrayUtil();
    currentUserGroups = arrayUtil.convertArray(currentUserGroups);
}

gs.info(currentUserGroups);
gs.info('Object type: ' + Object.prototype.toString.call(currentUserGroups));

Global:

*** Script: 723aa84f5ba02200502f6ede91f91aea,cfcbad03d711110050f5edcb9e61038f
*** Script: Object type: [object Array]

Scoped app:

x_149822_va_code_p: 723aa84f5ba02200502f6ede91f91aea,cfcbad03d711110050f5edcb9e61038f
x_149822_va_code_p: Object type: [object Array]

So with this simple solution the collection of groups can be handled as a JS Array in both cases.