An important thing to note is that at the same aperture setting (say f/5.6 for example purposes only) then DOF (Depth of Field) will be considerably greater at a focal length of 18mm as opposed to 200mm although it is better to avoid either extremes. Another point you need to take into consideration is, will the group be standing in a straight line parallel to you or literally as a bunch and how many will their be.
Here's a link to an online DOF calculator - just fill in the relevant boxes and compare the results. This at least will give you some idea of the figures involved and with your envisaged group layout should go a long way in answering your question. You might also like to consider downloading and printing out the paper version, laminating it and stuffing it away in your bag somewhere for instant off-line reference.
http://www.dofmaster.com/dofjs.html