Purpose should be important enough and meaningful enough to the team to spark their interest. If the goal has no meaning to the members and is not worth their effort they simply won't engage.
There should be rewards associated within to encourage active participation. The goals may be social, financial or personal in nature.
The objectives should be clear and people should know what success looks like. If achievement is not defined you won't have significant misalignment of action and possible motivational loss.
Each member should have a role. If roles are not defined many times people are not sure how they fit within the team and whether or not their actions are beneficial.
Members of the team should be treated as important participants. It isn't about making their boss look good or encouraging the glory of someone else. People deserve recognition, respect, and be treated as a valued member.