I've been asked to join in with a project and there is no actual leader (it's not a business setting), worse still there are about 8 members and because of this, there is no actual action plan. I'd volunteer myself as the leader or manager but I don't have the time or inclination to do so, should I just watch it struggle (and possibly never even take form) or is there something more constructive I can be doing?
Many projects can exist without a leader per say, but usually there is some kind of leader anyway, even though nobody has a hat that specifically says Leader of this project.
If a person has authority, he or she will most often make decisions that others will follow, and thus be the leader, even though that person might organisationally be at the same level as everyone else.
On the other hand, if every decision has to be a team-decision and consensus is hard to get because of different opinions, etc. then the project does indeed need a leader and nothing else will actually suffice.
If you're unfortunate enough to work on the latter, I'd say you should strive to either make someone a leader, labelled as such or otherwise, or become one yourself. Projects that argues about every little detail invariably fails miserably.
As for the no action plan part, does this mean you don't actually have a project plan detailing the things the project should do, like bugs that needs to be fixed, features that needs to be implemented, etc.? If so then I'd say your project is doomed either way, and I'm not kidding. This should be your first undertaking. Projects that plod along with we're going to do stuff and finish at some point will never work.
self-organized teams is common for agile shops. why should there be a leader? why not make everyone equally accountable for the success of the project as a whole?
There is a vague plan of what should be achieved but exactly what that entails is actually a point of discussion.
Usually someone will emerge as a leader. Not formally. But someone will set the pace of the project. So I'd just let it go and see what pops. Since you are asking, it would probably be you. And don't kid yourself, most people want to be lead. Just like any group, there are rams mix in the herd.
And just because there is no action plan, doesn't mean you can't keep one.
generally, "lead, follow or get out of the way"
if you're not keen to lead, and there's no one to follow, i'd suggest you ignore the whole thing.