Two years in, things got serious, and Hipster and his girlfriend opened a joint local account for rent, groceries, and the occasional trip. Both names, either can withdraw everything. Practically speaking, it's shared.
Hipster figured "joint" meant one FBAR could cover them both, the way he'd heard married couples do it. Close, but no.
The real shortcut is a spousal exception, not a joint-account exception. Under the FBAR instructions (Part III), a non-filing spouse can skip their own FBAR only if all the accounts they'd report are jointly owned with the filing spouse, the filing spouse timely e-files an FBAR reporting them, and both spouses sign FinCEN Form 114a (Record of Authorization to Electronically File FBARs). The couple keeps Form 114a; it goes to FinCEN or the IRS only on request.
Every condition assumes a marriage. His girlfriend isn't his spouse, so the exception doesn't apply, however long they've dated. Each of them has a financial interest in the account under 31 CFR § 1010.350(e)(1), and each reports its entire value, not half.
So Hipster reports the joint account at full value on his own FBAR, next to his personal and company accounts. If she's a US person, she reports it on hers too. If she isn't, she has no FBAR duty on it, but her status doesn't shrink his.