I've only ever seen two together, it was Spring so "romance" must have been in the air, as they can be very territorial and aggressive toward other robins. You are so lucky to have seen a group of five.
I have a bird bath that I make sure has water in it and many times I have had parents bringing babies to drink and bathe, and poo in it sometimes. Robins, Blue Tits and Blackbirds have all done it but I have to fill it up again after the Blackbirds as they spray it everywhere they are so exuberant. Once I had a family of LTT's and that was magical.