What do tenors do when the baritones sing silent night at church? I am talking about a tenors who is not in the choir. Do tenors try to sing a tenor line or just refrain from singing this low notes? Or maybe tenors refrain from singing as it is too low?
The range of a twelfth is a touch more than a lot of carols, but what most 'singers' with a small range do is jump the octave, and either drop down for the high three or four notes, or up for the lower ones. Some will do it without even noticing.
If the accompanist is accomplished, he (or she!) may put a song into a more comfortable key. Others bash on regardless. There may also be someone who is tone deaf, and sings the whole song on a monotone. No key problems for them!
One option is for the singer to choose a note from the accompaniment chords, and sing his choice, or even vocally arpeggiate the accompaniment chords, but the singer will need decent sight reading skills and a strong relative pitch in order to even attempt pulling this off. That said, if the sheet music is done right, each note on each beat will harmonize with the other notes and the melody on that same beat. It will probably take some practice to get the hang of it, but it can be a lot of fun when you do.
Assuming that OP is referencing the arrangement of Silent Night brought up in a previous question, the problem appears to be that the pitches of the melody are too low for some (high) tenors.
In that case, and if the tenor wants to sing the melody as part of the congregation, Tim's solution is the most obvious: skip around in octaves as necessary. Many singers of any voice part will do this when necessary.
When in a congregation, it's also usually possible to sing harmony (e.g., the tenor line), though in some places you might stand out if you do so. If the tenor line for this particular arrangement lies too low in parts, you could even sing the alto line. (I've certainly skipped up to sing the alto occasionally in this arrangement of Silent Night, even though I'm a baritone, just for fun because it's low enough and the men's lines are boring. And if no one else is singing parts around you, it makes some good harmony with the melody.)