President Jon Aarons, head of policy Nigel O'Connor, and government affairs group chairman Lionel Zetter have been asked to appear on 18 September for what is expected to be the last day of oral evidence on the issue.
IPR members are expected to reiterate calls for better self-regulation, including a tighter definition of the special adviser role, making clear they are political appointees and should not directly instruct civil servants.
The Committee has conducted the inquiry amid a spate of scandals, particularly at the now defunct Department for Transport, Local Government and the Regions. A report is expected by the end of the year.
Aarons this week dismissed as 'naive' some journalists who move into government PR.
In an interview with epolitix. com he said: 'I've met people in government comms roles that come straight from the media. They are regarded as a "catch because they are famous from TV or from the national press (but) they come in with naive ideas about the management and the democratic accountability of their post.'