Here are some options, depending on the context:
Formal:
* A team of organizers
* A group of organizers
* A staff of organizers
Informal:
* A bunch of organizers
* A crew of organizers
* A gang of organizers (if they are working together in a lively or informal way)
You could also consider using a collective noun related to the specific type of organizing they are doing:
* A committee of organizers (for planning events)
* A board of organizers (for running a non-profit)
* A coalition of organizers (for a social movement)
Ultimately, the best collective noun will depend on the specific context and the tone you want to convey.