There is several aproaches i've seen.
1. Hackatons like Startup Weekend where normally 1/3 comes with an idea, They pitch and votes are done with 3 sticky notes each, posted on the idea each participant like best. Top teams are choosen, and team leaders recruit members on the spot. In Startup Weekends they typically come with an idea without any work done ahead of time. So everything on the idea is done over 54 hours. You can definitely go to one of theese hackatons as a backend developer.
2. The other aproach i've seen is hackatons where existing teams sign up, and only the best teams attend. Just like a mini accelerator. Here you'll normally come with ideas which is worked on alot more ahead of time.