The "competition band" is the top kids. The audition process is very informal and takes place continually during the season. We start evaluating kids on day 1. Kids who are on the cusp might move in and out of the competition band, especially when eligibility hits. Most of the band will be in the competition band.
The "Friday night only band" is the alternates. We write drill with an "alternate line" in the back. Where possible, it's an extension of the main form. Sometimes it's just another arc or line behind the main form. They of course play all the stands tunes and do everything else the band does on Friday night. Saturdays are optional for them. They can come with, help move props, load the truck, etc. Alternates who demonstrate their drive by coming on Saturday are often first in line to move to the competition band, or at least be at the top of the list next year.
When you see our competition show, you would never know that the alternate line existed on Friday night. The form works with or without it.
We rehearse all of that together. The alternates get just as much attention when setting and cleaning the form, at least at the start of the season. As the season progresses and we get closer to contest, we definitely pay more attention to the competition only sets and let stuff slide a bit on the alternate line. The last week or two before contest, we sometimes have the alternates only come to rehearsal certain days of the week, so we can focus on the competition band.
For us, the system works great. All of the kids are working, the kids who aren't quite there yet get plenty of reps to build their skill and get prepared to move up to the competition band sooner or later. When we have kids who are completely uncoordinated, we can put them in the alternate line and it doesn't cause visual issues in the show.