Because, if you’re a person for whom the performance of happiness is problematic, there’s no greater sustenance than bilious company. And, if there’s one thing I love most about my daughters’ school, it’s that, mixed in with the happy and capable parents, the people who pay their bills on time and whose children do well at sports and have great handwriting, there is a sizable contingent of borderline depressives, lovely, cranky, and to my mind beautiful souls, with whom I can on the darkest of days find solace and community.

Could you explain the bold text?

Thank you
a bit depressive
marginally depressive
at the limit of depression

A further comment.

borderline depressives - 'depressives' is used here as a noun, ie people who are marginally depressive.

Best wishes, Clive