Gonna be a contrarian and speculate that rather than joining the Paisen Bowl, Opai Senpai is going to berate him for being timid and not recruiting new members. From there Nagatoro gets jealous/pissed and jump out to defend him/her territory. Cue training/cultural festival arc where gyaru harem bands together to show up Opai Senpai.
Step 3 - Opai joins the harem
Step 4 - Profit