Social media can serve as a platform for collective engagement with diverse affordances during crises. We explore how social media served this role by focusing on how online mental health discourse evolved during the COVID-19 pandemic. Specifically, we examine shifts in collective affordance dynamics within the online mental health community using Twitter. A comprehensive dataset of mental health-related tweets from 2018 to 2022 was collected (N = 3,953,836) and analysed using Computationally Intensive Theory Discovery as a guiding methodology. A subset of 757 representative tweets were categorised into a cascading set of actor groups. Analysis uncovers that collective engagement transitioned from decentralised actor utilisation (pre-crisis) to centralised organisational utilisation (early-crisis), culminating in centralised actor utilisation (late-crisis). The study contributes theoretically to collective affordance knowledge by integrating dynamics in an online setting and practically by revealing key actors' evolution in shaping online discourse across crisis phases.