Tell them what to expect, a little about the event, directions, where to meet. This could be: "Just a meet at the pub, they have great music on Thursdays. It should be fun. Meet in the car park by the pub garden"
Or. "I love Chatsworth House, would anyone like to join me on a visit there? There's loads to do there, have a look at the link www.chatsworthhouse.co.uk."