I get on first and then stand the bike up. Then I tell the passenger to mount (my wife for example) she will step on the left passenger peg while steadying herself with my shoulders as she swings her other leg over. She knows to try to stay as close to center line as possible whilst doing this.
You have to be holding the bars firmly and anticipate the bike trying to lean to the left initially. But if your expecting it; it's not an issue since you are firmly correcting by keeping the bike upright with firm body control.
I also keep the front handbrake engaged as well while she is mounting the bike.