Hi,

Steps:

1. Settings->Technical->Automation-> Automation rules

model: Sales order(Subscription model is also sale.order)

Tigger: State is set to Sales order (When a sale order is confirmed, the related subscription is considered to be in progress).

2. Add an action for the automation rule.

Hope it helps.

Go to: Settings > Technical > Automation > Automated Actions
Create  a new automation Action

• Model: Subscription
• Trigger: On Update
• Trigger Fields: state
• Filter: State = 'in_progress'
• Action To Do: Execute Python Code
• python code 

group = env.ref('base.group_user') 

partner = record.partner_id

user = partner.user_ids and partner.user_ids[0]  

if user and group not in user.groups_id:

    user.groups_id += group