A farewell has two jobs, and invitations usually do only one
The first job is logistics: which evening, which room, how long you are keeping it open, and who is coming. The second is the part that makes it a send-off rather than a meeting, which is other people saying something. A colleague leaving after nine years, a friend moving city, a neighbour going abroad: everyone wants to add a line, and there is usually nowhere for those lines to go.
Put both on the page. The practical facts sit at the top, where guests read them twice, and the messages have somewhere to be collected, moderated and kept. The reply list is what you hand to whoever is ordering the cake.