>>25939807
In 2014 it was just like you said...Moa and Yui were away a lot, so they needed 2 extra bodies for TIF, as well as a drill sergeant type among the girls. That was Raura's role in dance practices when she was in the group, and Yui took over that role in 2014. With Yui out, they brought in Raura.
In 2015, various girls were brought in to help with getting the 6 new girls up to speed. Raura, Marina, and Hinata, maybe others.
In 2016 there was a problem that was mentioned in the behind the scenes documentary about the girls practicing for their play that year. The "Institutional Knowledge" had basically all graduated out of SG. Aiko was the only one with more than 2 years in the group. This was a symptom of having only 10 girls in 2014. So they brought in Raura and Marina to practice with the girls, take part in the play, but more importantly give the girls some "come to Jesus" moments regarding what it means to be a part of SG.
After that I'm not certain but it wouldn't surprise me if grads came in to coach here and there.