We used to organise all of our golf rounds using a big WhatsApp group. I admit it was a crazy system, but it was easy for everyone, except me, the organiser. I bascially ended up having to scroll through 100s of messages to see who was coming, who wasn't and who had dropped out.
I was shown I'm In by a friend and instantly saw how it could solve the problems of organising the golf, whilst also remaining really easy for everyone to say if they were coming or not.
We still have plenty of banter on the WhatsApp group, but we now use WhatsApp for what it's good at, which is messaging and save the event organising to I'm In.
For me the most useful feature of I'm In is the ability to copy old events to create new ones, as it saves me having to create a new event from scratch everytime.