See this post for a comprehensive guide to the Boarding Group system for RotR. Below is a step-by-step guide to getting a BG with screenshots.
Liners have reported great success in using the following procedure to obtaining a Boarding Group for Rise of the Resistance. This is for Disneyland but it works identically at HS in WDW as well
Step 1: Enter the park through the turnstiles prior to published park opening. Make sure your admission is scanned (not just waved through by CM).
Step 2: Once in the park, close out and restart your app (in iPhone, swipe up to force close). This will ensure that your App recognizes you as having entered the park by fetching your status from the server rather than relying on the cached data which may indicate that you still haven’t entered.
Step 3: About 5 minutes prior to published park opening, launch the app and select “Find out More” from the Home Screen.
Step 4: Synchronize your time to time.gov to make sure you are ready right on time.
Step 5: Wait for 8:00:00am (or whatever the park opening is) and if “Join Boarding Group” is orange (meaning active), click it! If it is not active, select “My Status”. My Status fetches data from the server so the app will be ready to issue a BG right when the virtual queue opens. “Join Boarding Group” appears to refresh periodically rather than on demand, so can be delayed.
Step 6: Select “Join Boarding Group” from this page if the button active (dark blue outline). If it is grayed out, click the back button in the upper left hand corner. If the button is still not active in the previous screen, click My Status again. If the button is still not active, refresh by pulling down the screen. If this doesn’t work, force shut the app and try again.
Step 7: At this point the app will ask you for your party, etc. It is recommended that you unlink tickets that aren’t going to be used so you don’t have to waste time unchecking tickets. If you have someone selected who is not in the park, the app will make you unselect them.
Step 8: If the app does not recognize a member of your party as being in the park, get a BG for everyone else in your party and then approach guest services for assistance adding the other person to your party. You will be much more successful getting someone added to an existing BG than trying to get your entire party into a BG once they’re all distributed for the day.
Special thanks to @Incredible_Family and other Liners on Chat for these tips!