[{_id: 1, groupId: 10},
 {_id: 2, groupId: 20}]

[{_id: 1, groupId: 10, userData: 'A'},
 {_id: 2, groupId: 10, userData: 'B'}]

[{_id: 1, groupId: 20, userData: 'C'},
 {_id: 2, groupId: 20, userData: 'D'}]

$lookup: {
    from: {
        $cond: [{
                $eq: ['$groupId', 10]
            },
            'group10',
            'group20'
        ]
    },
    localField: 'groupId',
    foreignField: 'groupId',
    as: 'data'
}