These two both require the most amount of players. and can be done in one 1 hour session. Very easy to get through as long as every player is co-operating properly.
This one is easy, we need 8 players in our lobby, each player in turns, will start up a posse and invite the rest. As soon as there are 8 players, Achievement done.
The Kingpin achievement in Red Dead Redemption worth 55 pointsFight off 8 unique players during your time on top during Land Grab in Multiplayer Free Roam. Kingpin requires a lot more communication and focus than Posse Up. However, The way I always do this one is to get every player to meet at a certain location (I will advise where), then in order of the session, take turns to pop each player while in the objective. There's only a few minutes before the objective is over, so we all have to re-locate on the map to another town and repeat.
If you need either one of these, get in touch and I will make a session. Or keep an eye out for sessions as I will be putting them up randomly depending on the demand.