From
https://www.westjet.com/en-ca/rewards/tiers/platinum under the Early access to premium tab:
- Early access to purchase an upgrade to Premium or Business is available between 48 hours and 24 hours prior to departure.
- Requests must be made through the Platinum priority support number (via the WestJet app) between the hours of 6 a.m. and 9 p.m. MT.
- Valid for member and up to eight (8) additional guests travelling on the same reservation, subject to availability. Not valid for companions when travelling on a group booking (10+ guests on the same reservation).