We only have three whitelisted factions on our server, and none of them are a border patrol group (though there are elements of it in a couple). There is a "border" marked on the map, but that isn't a border between two countries: instead, the border marks the edge of what our resident criminal cartel, Poseidon, consider to be "theirs" - something that the Altis Government, in the form of the police, do not agree with!
The closest we have to a border patrol law enforcement group is the Altis Police Service. The roleplay for this group is, as the name suggests, police officers and law enforcement. There are specialist units within the police that you can join who do work along the border. You can apply to join them here: https://www.roleplay.co.uk/forms/5-altis-police-application/
On the other side, we have the Poseidon drugs cartel. Their roleplay is as an organised criminal syndicate who want to control all illegal activities on Altis. They are the ones who set up the border checkpoints, and deal harshly with anyone who crosses without getting a "border pass" - essentially an extortion racket where they will only leave you alone by paying Poseidon protection money. You can apply to join them here: https://www.roleplay.co.uk/forms/16-poseidon-cartel-application/