For coaches reach out to all communities with people that just have enough knowledge to be helpful for the learners. Be clear about that you don't have to be an expert, just know a bit more than the participants. It's ok for coaches to also learn at the event. Communities could be user groups, hubs, university IT students, ... Tell them how much fun and how rewarding it is to help others to learn tech and that they don't have to prepare or present anything, just be there and be nice. You can point them to the OTS blog where we have some posts about how workshops were with some nice pictures.