When our guild has a retreat several people have the large mats. These are set up so all can use. you can have a small mat at your sewing area. Irons are also furnished. We have 3 ironing boards and irons set up in an area at the end of the room.
In the case with my machine, I have small mat, cutter, rulers (for my project) pins, extra cutter blade, extra machine needles, bobbin or two. I can usually get the fabric and threads packed in around the machine. Oh yes, do not forget your machines power cord and foot control . Seems like every retreat someone forgets these.
Have a rolling suitcase for clothes and put anything that doesn't fit in with the machine in the suitcase. We are very casual at our retreats, so its jeans and a tee or sweat shirt. Jammies usually appearing early in the morning in the sewing area. Most do not want to wake their room mates if they are still asleep. Hope that helps a bit and have fun at your retreat.