Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- No one should pass in front of an Imam or someone praying alone.
2- The private zone for the Imam (sutrah) is sufficient for all people behind.
3- It is permissible to pass in in front of someone who joined prayer later because they are in a congregational prayer.
Answering your question about passing in front of someone who joined prayer later, Sheikh Kifah Mustapha, the Imam and Director of The Prayer Center of Orland Park and representative for Dar El Fatwa of Lebanon in the US, states:
Securing all the best of environment for the best of establishing prayer
Prayer is the main pillar of Islam after the testimony of faith. Prophet Muhammad (peace be upon him) said: “Islam is raised on five (pillars), testifying (the fact) that there is no god but Allah, that Muhammad is His bondsman and messenger, and the establishment of prayer, payment of Zakah, Pilgrimage to the House (Kabah) and the fast of Ramadan” (Muslim)
For that, securing all the best of environment for the best of establishing prayer is an important matter. So, this will help you in your focus and contemplation. Allah the Almighty said: {Certainly will the believers have succeeded. They who are during their prayer humbly submissive.} (Al-Muminun 23:1-2)
What Is Sutrah?
And for that important pillar and establishment, the Shariah secured a private zone for each person while praying so he or she will not be disturbed. Among such matters is the area in front of the person while in prayer referred to as (Sutrah) in Fiqh.
Prophet Muhammad said: “if anyone among you prayed, let him pray to a (Sutrah) and be close to it so Satan will not cut off prayer on him.” (Abu Dawud)
The majority of scholars said that it is preferable that each one puts an object in front of him during prayer (as a private zone) to protect the sanctity of his prayer. This means if the (sutrah) object is not available, prayer will still be valid but a private distance should be protected for no one to pass or cross in front of you.
This object is in a fair distance to protect your place of prostration (sujud) as described by the Companion Bilal ibn Rabah saying that: when Prophet Muhammad entered the Kabah, he prayed and between him and the wall is a distance of three arm length. (Ahmad)
This distance is around one and a half meters.
Passing in front of someone praying
Based on that, it was declared prohibited or preferred not (makruh) to cross or pass in front of someone praying in this short zone of prayer. Prophet Muhammad said: “If one passing in front of someone praying would know what liability (he is in,) it would have been better for him to stand forty instead of crossing” (Al-Bukhari)
Let me add that, if one is in a congregational prayer, the Imam’s sutrah is considered a sutrah for all behind him. Ibn Abbas said: I was young, and I came riding a donkey and crossed in front of people while Prophet Muhammad was leading prayer. I released my donkey and joined prayer and no one denied what I did.” (Al-Bukhari)
The Ruling
So, to answer your question directly, yes, it is permissible to pass in in front of someone who joined prayer later because they are in a congregational prayer. People in congregational prayer might need to leave prayer for all kind of reasons and walk between lines. They need not to worry.
The private zone for the Imam (sutrah) is sufficient for all people behind. No one should pass in front of an Imam or someone praying alone in the distance we explained above.
Almighty Allah knows best.