This guidance is quite long. To read it more comfortably you may want to print it off:
Although the new starter’s line manager will normally have the overall responsibility for the new starter’s induction, this does not mean (s)he will be the only person involved. Practice will vary according to departments.
Those involved in the induction other than the line manager may include:
In many cases the new starter may be allocated an induction colleague, buddy or mentor. There will be a number of considerations if you decide to allocate this role, including experience, expertise, personality and workload. The exact role may vary, but the individual is likely to be the new starter’s immediate point of contact within the department. As well as providing specific information and support with activities, if the new starter’s colleague has more of a mentoring role, (s)he may also provide more informal contact and information with the new colleague. This could involve helping to provide an understanding of the culture, the 'unwritten rules' and the way things get done in practice.
If the line manager involves a colleague / buddy / mentor in the induction process, it is important to agree on actual roles and responsibilities. As the line manager is normally responsible for ensuring that staff receive an appropriate induction, (s)he should be actively involved in the review of progress, although many of the day-to-day activities may have been delegated.