To be invited on stage you need to attend Hana's previous concert, when she invites Edwin to it. Then just choose to cheer for Hana, preferably with Mina.
For the Hana/Mina scene to play out rather than fail you can't have Hana as girlfriend, but need casual relationship with both girls. Also, don't push Mina towards Veronica, because then she'll be too busy sucking faces to see what's going on between Edwin and Hana.
For the Hana/Mina scene to play out rather than fail you can't have Hana as girlfriend, but need casual relationship with both girls. Also, don't push Mina towards Veronica, because then she'll be too busy sucking faces to see what's going on between Edwin and Hana.