What group or denomination does Jesus describe in Matthew 7:21 ('Lord, Lord')?

It is not limited to any group or denomination. Anyone who says, Lord, did we not do these miracles in your name, could be a lone charismatic figure gathering his own following in his own name, claiming to do miracles and works in Jesus's name. It is a general statement that applies to anyone in the world who says Lord, Lord, and Jesus will say, depart from me, you workers of iniquity, I never knew you. As 2 Timothy 4 says, the Lord knows those who are his, so from the beginning he knows, and no one will fool Jesus with false credentials. So the answer is that it does not apply to anyone specific; it applies to everyone.