...

A guess, without more to go on. It could refer to decision-making structures involving members of groups and a shared process going beyond groups.

I would have initially thought that the first was perhaps decision by "adherence to a party line" and the second shared decision-making, but after looking at where the text seems to come from ("Les Grandes Mutations de notre société"?), the two notions don't seem to be opposed but rather presented as equal manifestations of a transformation of power... The text below suggests a "decision par adhésion (consensuelle)" is one made by a show of hands, ie showing adhesion to/acceptance of a moved proposition, which perhaps sits alongside "shared" (or "participatory") decision-making, the latter involving a more active process? (eg. in formulating the options?).

Just a guess...

"Les propositions des ateliers sont transformées en décision par adhésion consensuelle en plénière (utilisation des signes d’accord en agitant les mains)"
http://fsl33.apinc.org/bb/viewtopic.php?t=50

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Note added at 2 hrs (2008-11-04 10:55:51 GMT)
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Or perhaps "decision by agreement and shared-decision making"
Cf. "Toute décision, qu'elle soit prise par l'assemblée des créanciers ou
obtenue par adhésion subséquente, doit faire l'objet d'un acte authen-
tique..." (www.admin.ch/ch/f/rs/2/221.522.1.fr.pdf)
Seems to be a term used a lot in discussions of assemblies, committees: in context I'd still say it's the difference between being actively involved in making decisions and decisions depending on obtaining agreement, as opposed to eg. unilateral decision-making.