You can name those who jump in quickly, volunteering to tackle any new task their supervisor proposes. (“Sure, Boss, we can do it.”) And you can name those who are more cautious, evaluating the time commitment and resources.
I’m thinking, too, of all the people who join a committee and then fail to follow through.
Or the coworker who’s muttering under his breath once we’re overloaded with the additional project. Or all the other tasks that get slighted.
How many others do you know?
Jnana's Red Barn 
Aren’t humans wonderful in our variety and complexity? Committees and work groups, on the other hand, are a very different kind of animal. They seem to draw out archetypes which aren’t always effective. Besides the yes-man, flake, and studious ponderer, it’s likely to see a distracter, team-builder, philosopher, back-slapper, comedian, resentful drudge, social butterfly, or lost soul.
Then again, those are roles, and since people are more complex and interesting than the personas they project, it’s possible for members to go beyond and become more human.
Vincent
And the plot thickens!
Your encouragement, I should add, is most welcome.
Thank you, and thanks for your inspiring thoughts and questions.