The first caller to execute this XML joins the conference “My Room.” When a subsequent caller executes this XML, they’ll join the same conference and the conference will start. 
from plivo import plivoxml

response = plivoxml.ResponseElement()
response.add(plivoxml.ConferenceElement('My Room'))
print(response.to_string())

# Or, you can use add_conference
response = plivoxml.ResponseElement()
response.add_conference(content='My Room')
print(response.to_string())
Response
<Response>
  <Conference>My Room</Conference>
</Response>