To assign a co-organizer role to an existing meeting participant:

1. Open the meeting from the Teams Calendar or Outlook.
2. Click Edit to modify the meeting details.
3. In the “Add co-organizer” field, select the participant you want to assign.
4. Save the changes.

If your question refers to a different scenario, such as assigning a co-organizer after the meeting has started or to a user without an account, please clarify so we can assist further.