Jealousy vs Envy
"Envy (also called invidiousness) may be defined as an emotion that occurs when a person lacks another's (perceived) superior quality, achievement, or possession and either desires it or wishes that the other lacked it." --wikipedia.
"A man might experience envy of another man who possesses what he wants, but lacks. The husband, however, may be jealous of his beautiful wife if he suspects she is developing an interest in another man. Envy implies covetousness, malice, and ill-will directed at someone who has what you lack; jealousy, in contrast, implies the fear of losing to a rival a valuable partner that you already have." -- David Buss
Please read the above until you feel the distinction.
You are not experiencing jealousy unless you are fearing a potential loss.
You have to have a relationship first.
In this case, your jealousy is directed at her and concern is with your efforts to "keep" her.
What we observe far more often in the community is envy.
1. Guys are envious of other guys because of the women, the lifestyle, the skills, etc, that they possess.
2. Guys are envious of women because of their apparent ease at attracting lovers, both men and women.
(In our society, vagina envy is far more prevalent than penis envy, in my observation.)
Any talk of what to do "when more than one guy likes the same girl" is usually one of envy, since:
1. The girl has no obligation or commitment to either guy.
2. The guy's animosity or frustration is directed at the other (rival) guy, not the girl.
Now, remember I did say fear of a potential loss. That doesn't require a real relationship, only the possibility of creating one.
Here's the thing. Guys will often feel jealousy first in a situation when they realize they have lost a potential connection (and any prior effort/investment was for nil).
Then, notice how quickly his jealousy ('loss' of her) turns to envy (ill-will directed at the him who caused the 'loss').
This is why ideas like 'women belong to the community' and 'they're all my women' don't sit well with scarcity-minded guys.