The deal, announced this week, values Clear - the firm run by former Labour Party comms director John Underwood - at just under £1m.
Underwood was Labour's top PRO from 1990 to 1991.
Clear specialises in public sector work, mainly handling NHS projects.
Clients include the National Clinical Assessment Authority.
Underwood and co-director Lesley Sargeant join Sheila Gimson, Tony Martin and Mark Westaby as directors of the Portfolio Group.
Portfolio mainly comprises B2B and IT PR firm Portfolio Communications, and media analysis and research firm Metrica.
Westaby said: 'There has been enormous investment in the public sector, which a lot of our clients are looking to get into. Clear specialises in crisis management, where there is a growing amount of private sector work. There's also a lot of crossover with public-private partnerships.'
Within the past month, Portfolio has set up The Answer, a unit to target firms with PR budgets of around £1,500 to £2,000 per month.