I think Anya and Tara were a part of the Scooby gang. No one made them feel unwelcomed. Xander had brought Anya to all the Scooby meetings since they started dating. Willow had always included Tara, and made sure she felt like part of the team in Real Me. It's not Buffy, Xander, Willow and Giles' fault that they knew each other long before they met Anya and Tara. They had a history together. Of course, they'd be tighter than with the others.