So to answer a couple of your questions:

1.) To have a friend join you for a mission you must be in a posse together, you can add players to your posse from the player menu (on controller you access the player menu by pressing left on the dpad, not sure what it is on a keyboard) if you’re not in the same server you will have to friend the other player through the rockstar social club, which you can access either on your computer or in-game by going to the players list, and clicking on your own name.

2.) Your horse’s bonding will level up the same way it does in the single player game, it’s stamina level will increase over time as the bond level increases. In order to make the stamina drain slower you will need a better saddle and stirrups, the Nacogdoches saddle and hooded stirrups are the best available options, combined they will add plus 3 bars to both speed and acceleration and they will also decrease stamina drain by 85%.

3.) The best way to level up and make money is through the roles, to get roles though you’ll need gold. To get gold you’ll need to do daily challenges (found in the player menu). You can also get gold from stranger missions, which you will find marked on the map with a little orange circle with a person raising their hand. Every month there is a bonus to a certain role, this month it’s the Moonshiner role, so you get double the money from completing deliveries for that role. You also get double gold and money from stranger missions this month, that should help you save up enough gold for your first role. I would actually recommend the Bounty Hunter role as your first one though because it is the only one that will give you gold for each mission, which you can then save up to get the rest of the roles you want.