Some ticket agencies also offer a 24hr/7day telephone credit card service. The larger agencies have an allocation of seats at each performance, so that if the theatre Box Office is sold out, they may be able to help you. If you book with an agent, they should tell you the face value of the ticket, the location of the seat, and if the view of the stage is restricted in any way. They will generally charge a service fee, but reputable agents will advise you of this at the time of booking, and it will not exceed 25%.
An alliance of reputable agents called STAR - Society of Ticket Agents and Retailers - works on a similar basis to the ABTA travel agents association. It has a code of conduct by which all its members must abide. You are strongly advised not to deal with anyone who is not a member. All agents will handle group bookings, some make it their speciality.