Suppose the AE / BE terminology difference over line and queue is ignored.
According to native speakers ( and google searches), the following:
He joined the lunch line.
is standard English. But then, I found this:
He entered the lunch line.
Google searches suggest that "enter a line" is less common than "join a line". So, is "enter a line" formal English?
When I was a child fifty-ump years ago and actually stood in line to get my school lunch, the expression was invariably get in the lunch line.
Join a line is an acceptable variant, which avoids the informality of get. (But I doubt that get will be unacceptable in any register if what you're speaking of is lunch lines.)
I have never encountered enter a line in this sense, and a quick Google suggests that it is used primarily of military units taking an assigned place in a line of battle, or of entering—keyboarding—a line of text into a field. It is odd in the context of a queue: it suggests ‘breaking’ into the line, not taking a place at the end of it.
Stick with get, or join if get makes you uncomfortable.
