Gotcha, so glad I don't have to go through a paywall like MG1000 (Monster Girl 1,000) and UvU (Unveiling the Unknown) does for their shared discord, really. ^^
EDIT: Also, finally managed to answer my own question. In order to have Evie available as a party member, simply do the story event in regards to encountering the Demoness for the first time. After that, you can invite her into your party from then on. Still have yet to know how to have Galiene become a possible party member yet though.