You may invite anybody to join the worship of the Mass with you.
I would not limit that invitation solely to those who have been baptized. Invite anybody who may be genuinely interested in learning more about the Catholic Church.
You may need to sit in the back of the church in order to explain what is happening to a person who has never been to Mass before.
The non-Catholic churches that I have visited do not have kneelers. As already mentioned, there are physical reasons why a Catholic might not kneel. Sitting is the least noticeable posture to take when unable or unwilling to kneel. Yes, do be aware that the person behind you may want to kneel.
The non-Catholic may merely be an observer, again making sitting the proper posture to take; and of course standing when everybody else stands (unless there are physical limitations).
As Catholics, we may likewise attend the services of our non-Catholic brothers and sisters. Remember of course that we must first attend Mass. As our non-Catholic brothers and sisters may not receive Communion in the Catholic Church, neither are we to share in the “Lord’s Supper” offered in a non-Catholic church. Do let your guest know this in advance. I do not consider the comfort of my guest a legitimate reason to refrain from receive Communion, “the source and summit” of our faith.
I attend a Mass in a large parish where readings and prayers are projected on the front wall. One of the slides includes the statement Communion is to received by Catholics only. Since the Masses for this large parish are generally packed, I might even sit in the courtyard with a first time guest in order to explain what is happening on the video screen.