Sarah crashes a party hosted at the Dyad Institute in episode 2x01 "Nature Under Constraint and Vexed". From episode 2x02 "Governed by Sound Reason and True Religion". From episode 2x07 "Knowledge of Causes, and Secret Motion of Things". Appears in 3 more episodes.
Source: Orphan Black — Wikia
Sarah takes Kira to a pediatric hospital to donate her bone marrow for Cosima in episode 2x09 "Things Which Have Never Yet Been Done".
Donnie takes Helena to the hospital to get her first ultrasound in episode 4x02 "Transgressive Border Crossing".
Mary helps a stabbing victim and gets a pair of referrals from Annie in episode 1x01 "Bloody Mary". Annie identifies another terminal patient in need of Mary's services in episode 1x02 "The River Styx". Mary meets with Annie to give her the money for Irene's case and reassure her about the cops in episode 1x03 "Wave the White Flag". Appears in 6 more episodes.