I suggest jumping off the tower and killing yourself. Seriously, it shows up in the screen that you died. There's your "shout".
If it doesn't yield results, go to orbit and come back down. Hopefully you get a new instance of the tower and can kill yourself again for notice.
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The bottom line is this. There needs to be some universal way for someone who needs a raid group to signal this in game. I remember way back in the early days of DCUO before dueling, heros and villains would meet in the PVP phase, atop Star Labs, and stand around. When someone wanted to duel, they would walk to the middle and wait for someone of the opposing side to challenge them. The signal that both were ready was that they would both bow. I'm not suggesting that killing yourself be the end all be all solution, but if you want to make it easier for people with minimal friends online, then we need a common signal. Checking the forums isn't always an option.