May I suggest that collective action for degrowth needn't -- and probably shouldn't -- only include activism directed toward degrowth per se. In most places, there are quite a number of other things that have to change in order for degrowth to be implemented. (To clarify: I'm speaking in this post mainly about implementing new public policies, in contrast to private initiatives such as voluntary simplicity.) 
For example, where I live, there are such diverse needs as (i) improving labor arrangements and support systems (day care, etc.) so that working people will find it easier to raise families, (ii) avoiding or even pulling out of certain free trade agreements, and (iii) making the electoral system more democratic. The coalitions supporting each of these propositions will often be quite different, even though there may be some overlaps. The constituencies who must be persuaded in order to change them will also often be different -- sometimes citizens, sometimes employers, sometimes bureaucrats, sometimes politicians, sometimes a mix of these and others. 
When you target degrowth directly, you are often aiming only for an overlap constituency among citizens. But you can't make degrowth more than a slogan unless you get these local, heterogeneous obstacles out of the way. That goes for private initiatives, too: they may have little or no effect if public policy is pointed in the opposite direction.
So among the tasks for activists are (i) identifying these roadblock policies, and (ii) creating an action plan for removing them. My own point of departure in approaching these issues is that degrowth is most likely to be implementable at a country-by-country level, and that the roadblocks will vary a lot according to a country's specific situation. I know little enough about Europe to be at best agnostic about the chances for success of a Europe-wide movement; to my outsider's eyes, Germany, Greece and Bulgaria, for example, seem to be quite differently situated. But obviously, the more transnational the level at which you want to implement degrowth, the greater the number and complexity of the particular obstacles you need to assess and remove.