That's my granny, Violet, in the bottom right hand corner. I'd know those eyes anywhere. Her sisters Kathleen and Ethel are sitting at the far left. I think Joan (dear Aunt Joan) may be the blonde kid in the middle.
They were the Bridges girls. Their father had a shop (millinery, I think) in Maidstone, Kent.
I don't know what's going on- amateur theatricals? a fancy dress party?- or who the others are. The date must be round about the outbreak of the First World War.