The publication of a photograph, for commercial purposes, is a bit of a grey area when it comes to considering whether the permission of those in the photograph is required. Photographers are sometimes advised to get those in the photographs (or their parents, in the case of minors) to sign 'model release' forms to avoid any element of doubt. However, this is not obligatory. The publication of such photographs (with or without a model release) would never breach any criminal laws. If the person in the photograph believed that they should be paid for the use of their image (i.e. they've been used as a 'model') they could sue, through the civil courts, for payment but, in practice (unless they've got a famous face, e.g. David Beckham) the amount that could be claimed (if anything) would be tiny (�20 to �100 perhaps?) while the costs of bringing such an action could run into tens of thousands of pounds.
If you're planning to use shots, taken at your outdoor pursuits centre, it would probably be wisest to ask permission from the group leaders, purely as a matter of goodwill. (i.e. it's extremely doubtful that the use of the photographs would bring any legal problems but it would obviously be unwise to upset your clients if you want them to bring more groups to your centre).
Chris