Forgot to say what is critical. First, comfortable chairs. Retreat center in West Union, Iowa had rolling chairs for each of us. One table per person with some extra for exchanges, snacks, etc. Even better to have a couple of tables and chairs set up like a dining area for relaxing away from the sewing area. A couple of rocking chairs again to get away from sewing for awhile is nice.
Cleanliness of bathroom facilities is a must. Some privacy if the facilities are shared is needed. Ladies generally don't want to shower all together.
Good light is needed.
Shared irons and ironing boards are better than many around the room because of electrical loads. (with 22 sewers, we needed three of each) Depending on the size of the group, one or more cutting areas at the proper height really help. Design walls are a big plus. Don't usually have these. Holly Girls retreat is able to hang flannel backed vinyl tablecloths on the walls.
They also got coffee cups from Goodwill and each person used one for the week then they were washed for the next group. No one wanted to waste Styrofoam cups. Coffee and a refrigerator for cold drinks was needed by all.