All lenses, of all focal lengths, have the same depth of field at the same aperture and magnification.
The rule of thumb is: there really is no depth of field, just progressive deterioration which, with digital, seems to be even more marked. The rule printed on the other thumb tells you to focus a third of the way into the scene, on the principle that there is more imaginary depth of field behind the plane of focus than there is in front of it.
Solution? Try multiple shots, if static scene, or stick the people fairly close together and pray. Film would help because you could stop right down with less (?) deterioration.
In an ideal world, that's where camera movements would help you out.
Rob C