If you're fortunate to be included in the roster to help build a sustainable building, use your common sense;
No one who has spent months (or more likely years) plotting and planning for their very own little sustainable house will appreciate someone spouting off about how they would do it.
If you're an experienced builder, then your point of view will be valuable, but don't expect to make changes to long standing plans. Allow the owner builder to take the stage, and don't try to upstage them.
If you're not experienced in any kind of building, then it's best to just watch and learn. There is always clean up to do, moving cut off ends of lumber, sweeping up sawdust and generally removing tripping hazards.
If you have special dietary needs, don't expect the hosts of the work party to be able to provide for those.
Generally, kitchen facilities are minimal, especially on a site that is going to be the primary residence.
Lunches on my sites are most often cooked over an open fire, and tend to the hotdog and bun type of fare. Most of the volunteers, after a busy morning of activity, relish the simplicity, and being ravenous, appreciate the speed of the preparation of it.
Water, coffee or soft drinks rehydrate them to get them ready for the afternoon shift.
Parking can sometimes be at a premium.
Don't block the driveway so that important deliveries can't get through, or the owners vehicle in case of a supply run to pick something up. It can hold up the entire process to jockey cars in and out.
Once the days work is done, stick around for a few minutes and help tarp things up, move tools under cover and generally button things up. It will be greatly appreciated by the tired builders, plus it makes it much easier to get started the next day, with a clean slate.