[{"identifier":"b553622e-b1c9-4030-8460-a7783214359b","participants":["p1"]},{"identifier":"919f77bc-adca-4cbc-ac38-9139552cdb5b","participants":["p2"]},{"identifier":["919f77bc-adca-4cbc-ac38-9139552cdb5b","b553622e-b1c9-4030-8460-a7783214359b"],"participants":["p3"]}]

db.collection.aggregate([{$match:{$expr:{$eq:["array",{"$type":"$identifier"}]}}},{"$unwind":"$identifier"},{"$unionWith":{"coll":"collection","pipeline":[{$match:{$expr:{$ne:["array",{"$type":"$identifier"}]}}}]}},{"$unwind":"$participants"},{$group:{_id:null,allId:{"$addToSet":"$identifier"},participants:{$addToSet:"$participants"}}},{$project:{_id:"$allId",participants:1}}])