We have a 'brew up' most Monday to Friday mornings, sometimes in the afternoons as well, we have a summer BBQ; we have coffee and cakes for those helping at our working parties (held two or three times a year), and we hold an 'at home' at our field shed (about the end of September, for those people who have taken on a plot recently, or are on our waiting list and near the top so about to get a plot, so they can see the range of equipment available to them on loan, and to meet the committee members) when we serve coffee and cakes. We also have an annual open day.
As for furnishing your empty communal shed - we asked plot holders for donations of surplus and unwanted chairs, cushions, crockery etc and a local carpet shop gave us a really good quality carpet (used but had been partly damaged and was replaced under insurance for a customer). Local skips are also a good source of items to furnish your shed! Also freecycle.