We were keen to make sure we weren't diluting the achievement based elements of gaining a badge by simply issuing them to whoever turned up to an event.
For our pilot badges we decided to make sure the participant had to actually participant in someway to the event or course.
For example, the students who took the e-Portfolios for the reflective learner course received there badges shortly after completing the course. During the course the students had to complete a short number of tasks and submit a final portfolio. This was their activity or task to prove that they had participated in the course.
The sentiment that the badge should prove that the participant has been 'involved' in their learning by proving active communication, teamwork or critical thinking was echoed in a recent Edudemic post 'Why The Future of Education Involves Badges' (http://www.edudemic.com/future-of-education-badges/).
"Articulating benefits from general education and extracurricular activities. For general education courses, colleges can use learning design principles to define “soft skill” outcomes and then measure competency against these objectives. Students who demonstrate they have acquired these critical job-ready soft skills will earn job-relevant badges in areas like critical thinking, research, oral and written communication, collaboration, leadership and teamwork."
This is why we advise NOT to simply issue a badge based on lets say for instance simply attending a workshop or conference.
We would suggest, instead, offering up a badge for anyone who wishes to submit a reflective piece based around any key messages they have taken away from that workshop or conference. This ticks multiple boxes, firstly it gets the participant actively involved in their learning and secondly it provides a nice conduit for more in-depth feedback about said workshop or conference. It may well depend on what information you wish to capture however we would advise keeping any reflective piece to around 150-200 words.