I'm exploring the chapel and cant figure out how to stop the rest of my party following Astarion while hes sneaking around.
The two ways I get my team mates to stop following is to remove the person from the party that I want traveling alone, or enter turn by turn mode.
Removal method:
1- select someone else as temporary leader.
2- Right click on the person you want to go solo (Astarion in your case), then choose "Ungroup"
3- Now select the person you want to go solo and the rest of your team stays put where you broke them off.
When you're done and you want to rejoin the party you can do a couple things: either a- switch back to a player in the "group" and have the soloist "group" by right click, or have the group move towards the soloist and group on that spot.
Turn-by-turn method is where you click the timer in the center of the screen to voluntarily enter turn-by-turn mode. You move the soloist at your whim and have the other characters stand around and choose "End Turn" immediately. When you're soloist is done, leave turn-by-turn mode, and your party rejoins the soloist.
Edit: apologies if these have already been suggested. I didn't read the entire thread before responding.