Database
[{"_id":ObjectId("648fc2857edfa7ee4f1ce31c"),"userId":"1","groups":[{"_id":ObjectId("648fc2857edfa7ee4f1ce31d"),"groupId":"1","groupLeads":[{"userId":"4"},{"userId":"5"}]},{"_id":ObjectId("648fc2857edfa7ee4f1ce31d"),"groupId":"2","groupLeads":[{"userId":"2"},{"userId":"3"}]}]},]
Query
db.collection.aggregate([{"$addFields":{"groups.gLead":{"$map":{"input":"$groups","as":"g","in":"$$g.groupLeads.userId"}}}}])