Let assume you created sec_group1 group, in the "Parameter" section provide security group part for example:

parameters:
  security_group_id:
    type: string
    description: ID of security group for this instance port
    default: sec_group1

The in the server definition re-use the security group by get_param, for example:

  security_groups: 
    - { get_param: security_group_id }