Author(s) and artists
Authors are of two types – the strict authors, in the sense of actual writers and arrangers of the guide, and the others listed as authors due to their crucial involvement in the project as a whole.
Be wary of having many strict authors of your guide, unless the guide can be broken up cleanly into areas of different responsibility (e.g. see Modular Guides). Co-authors might help with particular plant groups or with the task of researching usage, or local names, or running field trials. Even then, a designated main author should be prepared and empowered to take responsibility for the guide as a whole, checking consistency of information and style, working with the publisher, keeping other authors to deadlines and so on.
Be very clear about precisely what various contributors to the guide are responsible for, what benefits they will gain and what it will cost them in time, money, prestige and job satisfaction.
A main author - researcher, plus a research or field assistant and maybe an artist is a reasonable team for a modest technical field guide. Clarify whether co-authors are committed to the project for its own sake and may well end up working more hours than paid if this is necessary to get it finished; or whether they are in it purely for the money, and in this case the obligations and conditions of payment should be very clear. It may be embarrassing to discuss such mercenary matters during the honeymoon of the planning phase, but this is important because the time pressures will only increase as the book approaches publication.
For complicated technical guides, especially where there are more than 500 species, it is useful to have a main general author, responsible for general design, keys, field descriptions and so on, and a second taxonomic author responsible for sorting out nomenclature and taxonomically tricky groups. For a very large guide to a complicated tricky flora (see e.g. Ducke project & guide (English summary)), it may well be necessary for a main author, more like an editor, to coordinate several taxonomic specialists for tricky groups,
Artists are obviously vital if you are to include drawings or paintings. A good graphic designer would be useful if you have large enough budget and they can work closely and harmoniously with your publishers.
The core team or author needs to be closely linked to the others with various interests in the field guide - either informally or through regular meetings – whatever the author and others are comfortable with.
A field guide should always be "user-friendly", but getting this right obviously depends on defining and then familiarity with the interests and aptitudes of the prospective users. These might well fall into various categories, or user-groups, a fact which will probably lead to compromises in the design of the field guide.
Representatives of various user groups, ages, gender, experience etc., ought to be involved with all phases of production, except perhaps publication, and for this purpose it may be appropriate for one or more key members of a focal user group an honorary or actual co-author.
You may end up working or corresponding briefly or extensively with many people in herbaria, or experts in other parts of the world or in your study area. These collaborators might be considered as authors, in repsect of their intellectual contributions. However, a line has to be drawn somewhere between authors and the other people they talk to, so if many people are involved and are only to be credited in the acknowledgements, it is useful to clarify this very early on in the project to avoid disappointment later.
For printed field guides, potential publishers, page designers, printers and vendors should also be met with early in a project, and regularly afterwards to help steer the book to an efficient conclusion.