you need to learn callback style or even better async/await style. much more easier to code.

but for your specific concern, i think you wanted to put the res.send one line up because that way you will call res.send AFTER the second query has executed.

router.post('/createGroupe/:userId', upload.single('file'), (req, res) => {
    let groupe = req.body
    // req.body containing the groupe title, description
    let userId = req.params.userId
    let groupeId
    groupe['image'] = req.file
    db.query('insert into groupes set ?', groupe, function(err, result){
        if (err) throw err;
        groupeId = result.insertId.toString()
        db.query("insert into groupemembers set ?", [groupeId, userId, 'admin'], function (err, result){
            console.log(groupeId)
            if (err) return err;
            res.send(result.insertId.toString())
        })
    })
})