First of all you don't need a participant model. This is the structure I'll use if there is some extra information I want to store in the meeting model. If not you can directly use has_and_belongs_to_many.
User
has_many :meetings
has_many :attending_events, through: :meetings, source: "Event" //for the participants
has_many :events // for the event organiser
Event
belongs to :user // The organiser
has_many :meetings
has_many :participants, through: :meetings, source: "User"
Meeting
belongs_to :participant, class_name: "User"
belongs_to :attending_event, class_name: "Event" // Use attending_event_id as foreign_key