For practice you don't need to check people off, just hit join on that screen when only your name is checked (on real day anyone w park pass will already be checked).
For alignment - at any time during the day, click Join from the at at screen. On the next page is an orange triangle with a message (or right now I tried and there's a small clock icon and a message about the 1pm open). Wherever that message is in the page, hover your finger and hold it there. With the other hand click back to the main screen and scroll it (without moving that hovering finger) so that the At At join is under the hovering finger. Now they are "lined up". Now look at your screen and try to get some visual cues as to where you should place things for practice so the first join button is there (maybe the at at eyes are a certain spot, or a line of text equal with your volume button, etc)
I tried this 'alignment' method at 1 pm today. Unfortunately, alignment can be off, if the Loading... spinner hasn't yet gone away, but second Join button is available. I was able to get through without waiting for Loading... to go away, too.