The appointments, which include Simon Bielby, a business director at MindShare, will make the team more than 60 strong.
Bielby, a strategic planner on MindShare's Nestle business, is joined by the communications strategist Bruno Martin, who was previously at the econometrics specialist Point Logic and Saatchi & Saatchi.
Other hirings include the former Optimedia and Media Planning Group communications planner, Ian Turnbull, and Josh Howdle Fuller, who was previously a media strategist at Walker Media.
Initiative has also boosted its consumer insight strength with the appointment of Jane Anthony, the research manager at Camelot, and Jessica Lowen, who joins from Viacom as a strategic researcher.
The agency recently lost a senior member of its communications planning team when the creative director, Lucy Banks, left to join ZenithOptimedia in a similar role.
Initiative's communications planning director, Tony Manwaring, said: "One of the challenges faced by the industry is to offer credible strategic advice that is relevant and of sufficient scale to deliver accountable results for our clients. It is all too easy to equate 'strategic' with 'media stunt' and that is not what we are about."
Manwaring rejoined Initiative last year after several years running his own consultancy. He replaced Tim Allnutt, who left the agency to join Naked Inside.