getUseTexture.py

import pymel.core as pm

def getUseTextureThisGrp(grp):

    if isinstance(grp,str) == True:
        grp = pm.PyNode(grp)

    shapes = grp.listRelatives(ad=True,type="mesh")
    files = []
    for i in shapes:
        sEngines = i.connections(type="shadingEngine")
        for s in sEngines:
            shaderList = s.surfaceShader.connections()
            for shader in shaderList:
                fNode = shader.connections(type="file")
                for tex in fNode:
                    files.append(tex.fileTextureName.get())
    
    files = list(set(files))
    return files