Database
db={Group:[{"users":[],"admin":["5eac9981c6a12c190814ec2d"],"pending":["5ead65e3cb971e31a4d1c538"],"_id":"5ead578c145003326cbceda6","name":"Group 1","__v":0}],Users:[{"_id":"5ead65e3cb971e31a4d1c538","name":"Patate","email":"patate@orange.fr","__v":0}]}
Query
db.Group.aggregate([{$project:{pending:{$map:{input:"$pending",as:"item",in:{$toObjectId:"$$item"}}}}},{$lookup:{from:"Users",localField:"pending",foreignField:"_id",as:"members"}}])