In ordinary (non-macro) photography, depth of field depends upon distance from the camera to the subject to a very large extent. The size or image magification also plays a role in the amount of depth of field; an 85mm lens shot at f/2.8 at 10 feet will give a shallower depth of field than a 35mm lens shot at f/2.8 at 10 feet--BUT, the telephoto will produce a fairly large image of a person's head and shoulders with a narrow angle of view behind them, while the 35mm lens will give a smaller image of the person, with more of an angle of view behind them.
In the way that most people commonly use lenses, with wide-angles being used to include wide angles of view and SMALLISH renderings of things, yes, a wide-angle, 35mm lens at f/2.8 will show more depth of field than an 85mm used to take a tight, magnified "telephoto" type of shot. However...if one decides to get really CLOSE with the 35mm lens, to create the same,identical sized head and shoulder portrait shot with the 85 from farther away...then the 35mm lens at f/2.8 will have basically the same depth of field as an 85mm lens shot from farther away...
This is an attempt to simplify things, for brevity. There is also background blur, which is different with telephoto lenses than with shorter lenses. On tightly-framed groups of people, even f/4 might be too wide of a lens opening to ensure that everybody is acceptably in-focus in a larger print; I'd start thinking about f/4.8 to f/5.6 as being the territory where there's a little bit more safety, rather than f/4. Again, DOF "depends", mostly, on how far the camera is away from the subjects.
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