It sounds like this is a portrait shoot then?
If it's going to be casual then you can't go wrong with dockers, blue jeans, neutral color clothing for nice exposure. I tend to be a minimalist for this sort of thing equipment-wise, so even though you can make some very nice elaborate umberella and reflector setups, I tend to just use fill flash in nice even open shade.
You may want to consider using a tripod - not that it's so dark you will need it for exposure, but perhaps for consistent posing and as a "visual focus point" for the group to concentrate on.
For metering I'm a big fan of getting a good INCIDENT reading - have done this for decades in the wedding work - and then just adding a splash of fill flash for sparkle and shadow illumination. I use a handheld meter for this with the dome, but you can get a very good equivalent relfected reading off the subject with the camera.
To do the above, pick a mid tone color on the client (grass will even work) meter off this, note the reading, set the camera on MANUAL, dial in/lock the shutter & aperture, and then with your flash on AUTO, have it output FEC -1, or -2/3 or -1/3 of light and note the results. You want enough to get rid of any racoon eye shadows, but NOT so much that you give everybody the nuked-in-the-face look. You want to retain some pleasant modelling.
You also want a shutter speed that will be low enough to allow ambient background light (assuming shade environs) to come in pleasantly and not go dark or black. Hope this helps - let me know if you have any additional questions. - Stu