MongoDB Aggregation join array of strings to single string

You were on the right track.

Just add $reduce over $concat in your $project stage.

'collection2': {
    '$reduce': {
        'input': '$collection2',
        'initialValue': '',
        'in': {
            '$concat': [
                '$$value',
                {'$cond': [{'$eq': ['$$value', '']}, '', ', ']}, 
                '$$this']
        }
    }
}

Note: We use $cond to prevent a leading , in the concatenation. You could also use $substrCP before $reduce as an alternative to $cond.