The sooner you get your team together to do some things, the better it will be. Your field kit will arrive in August sometime, so if your team already knows how to work together, building the field will be an extension to your team-building activities. With the younger grades, even those with experience building Lego models will not have very much practice building with Technic parts. I think a good investment will be a medium-sized Technic set, truck or crane or plane. Build the model together (practice in following instructions) then build the alternate model together. Then build something else from those same parts. The good news is that all of the parts from that set or sets can be used for your FLL robot,
Once the team is comfortable building together, unleash the excellent site
nxtprograms on them. Dave Parker has put together an amazing (and free) collection of models to make using NXT parts. If you cannot find something to interest the team on that site, you should change over to gardening, or horseback riding or something. You could also support Dave by spending $20 for his CD that has all the same stuff as his site on it so you don't have to have a WWWeb connection for showing the stuff. Oh, and he also has beginner level programs for most if not all of the models.
Short answer is - spend as much time together as you can and don't worry about not having absolutely all the team at every meeting.