First, is this for Game Maker or RPG Maker? Because if this is for RPG Maker, the topic's in the wrong place and you probably won't get much help in this section of the site.
Second, do you mean how to switch party members out and into the party? If so, I'd use arrays. Have a global array for each parameter you use for a party member.
For example:
global.attack[1] = 10
global.attack[2] = 20
That code would set the attack of the first party member to 10 and the attack of the second party member to 20. And in your case you'd want this:
global.level[1] = 3
global.level[2] = 4
This would set the different party members level to 3 and 4 respectively. If you need more info on arrays, I suggest you take a look at my pin in this section.