Currently only the team founder can join a challenge. Sometimes the founder is mia or a configuration problem prevents the founder from accessing the team founder section on BOINCstats.

I propose that a team can join a challenge when a certain number of team members wants to join it. This can be a percentage of the total active team members (example: 120 active members, 10% threshold, 12 should want to join) or a fixed number (this is difficult since there are very different sizes of teams out there).

Showing intent to join would be as simple as clicking a button on the challenge page.

Thoughts, suggestions or concerns in the replies please.

Typically a quorum is a majority:
quo·rum (kwôrm, kwr-)
n.
1. The minimal number of officers and members of a committee or organization, usually a majority, who must be present for valid transaction of business.
2. A select group.

However, Willy's suggestion for a 10% threshhold for joining a challenge sounds reasonable. I'd suggest even a maximun quorum of 15 (that's the number of concurrent scouts that bees use to determine their new home) ref: http://www.news.cornell.edu/stories/April06/swarm.quorum.ssl.html. This maximum would help make it easier for smaller parties of a larger team to join in a challenge.

I'd also suggest awaiting discussion from a quorum of team founders.
Dan