Verse 1: “tax collectors and sinners”: In 5:30, “the Pharisees and their scribes” ask Jesus “‘Why do you eat and drink with tax collectors and sinners?”. [ NJBC]
Verse 2: “grumbling”: Or murmuring. A midrash on 2 Chronicles 20:37 counsels against associating with the godless. The whole chapter opposes this rule. [ BlkLk]
Verses 3-7: A possible parallel is Matthew 18:12-14, but there Jesus says “‘if he finds it’” rather than “‘until he finds it’”. Luke does emphasize all-inclusiveness. [ NOAB]
Verses 4-7: In John 10, Jesus also teaches using the sheep metaphor. In Ezekiel 34:11-16, sheep are a symbol for God’s people; he is the shepherd (as here). [ NJBC]
Verse 4: Comments: first-century shepherds were considered lawless and dishonest: unlike Old Testament shepherds.
Verse 5: “rejoices”: The theme of joy suffuses this chapter: see also vv. 7, 9, 10, 23, 29, 32. Joy in this chapter has four emphases:

The motifs of universality, community and soteriology (systematic interpretation of Christ’s saving work for humans and the world) are inextricably commingled.

Conversion is a prerequisite for finding joy.

Happiness consists in a willingness to share in God’s own joy in dispensing salvation.

The call to participate in God’s love and joy is issued through Jesus.

Verse 8: “coins”: The Hebrew word (and perhaps the Aramaic) for coins, zuzim, can also mean those who have moved away, departed . Perhaps Jesus uses a play on words; perhaps the lost coin is representative of those who have lost faith but can still be reached.
Verse 8: “light a lamp”: A Palestinian house had a door and no windows. The woman would “sweep the house” hoping to hear a tinkle. [ NJBC]
Verses 11-32: A third parable on God’s joy at recovery of the lost, the parable of the Prodigal Son. In this case, the father’s rejoicing at the return of his errant son has to be explained to the son who followed tradition (as the religious establishment did) and stayed at home. [ CAB]