Principals say the scheme will give students the widest possible choice, protect minority-interest courses and help colleges to benefit from economies of scale.
The group's marketing campaign includes common advertising - featuring a mock-up of a CD cover - and recruitment fairs at four of the colleges - City of Bristol, City of Bath, Norton Radstock college and Weston college, Weston-super-mare - at the end of this month.
The idea of joint marketing came at a meeting of principals earlier this year - the others involved are Filton College of Care and Early Education, Soundwell college and St Brendan's sixth form college in Bristol.
Geoffrey Terry, principal of Norton Radstock college, said: "Since incorporation we have tried to work together in many ways. And I think what's happening now is proof of how beneficial it is to have a common approach to certain activities."
Rick Dearing, principal of Filton college, said that an area-wide approach would benefit students. He said: "Previously, if a college found that there were so few students applying for a course it was no longer viable to run, it would probably have had to close. Now, through this collaborative approach, colleges can establish the best use of current resources and ensure that students will, in virtually all cases, be able to follow the course that they choose."
But Ursula Howard, of the Further Education Development Agency, said the colleges needed to give the strategy time to develop.
"Marketing is about changing assumptions, which can take a long time. Community colleges in America, who adopted marketing earlier than the UK, find it takes five to seven years. I would encourage those involved to develop a long-term commitment to the project, to maximise its potential."