Just use a Variable and change it whenever you change the Party leader.
Whenever you change the party leader you'll need to:
Control Variables: [Party Leader] ==(set to) 2
or
Control Variables: [Party Leader] ==(set to) 3
etc.
For the guy that you're talking to, make Conditions for each Party Member.
Conditional Branch: (if) Variable: [Party Leader] == 1
Show Text: Hi.
else
Conditional Branch: (if) Variable: [Party Leader] == 2
Show Text: Go away.
else
etc.