It's not 90 minutes before they open the entrance/viewing area. What happens is your return ticket has a hour long window, and when you arrive before that window(which you should) you are lined up outside the viewing area. In total aim for about 15 minutes before the window, 75 minute wait in total.
In that case of the ticket in the picture, I would aim for finding the RED line at 7:45.
As far as staying together. Assuming you have a cell phone it should be easy to two people to hold the spot for two more. It does get crowded, so it may be better to all go in, establish your spot. Then let them leave. It would be real easy to ride LM, grab a snack or use a restroom. Just make sure you have your show pass to get back into the viewing area.