Muster doesn't work correctly on Gemp. Players are given the option to use Muster on one character, discard one and draw one, then they get the option to use Muster on a different character.
The rules state that you must use Muster all at once; if you have 3 guys with Muster, you choose to use Muster 0, 1, 2 or 3 times, and then discard that number to draw that same amount. After that, you're done using Muster for that regroup phase.
Bib is right. From Comprehensive Rules 4.0:
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muster
At the start of the regroup phase, for each of your
characters who has the muster keyword, you may
discard a card from hand, then draw a card. You
must discard all at once.
If you have two muster characters, you cannot choose
to discard a card, draw a card, then discard another
card and draw a card. You must instead choose
to discard 0, 1, or 2 cards, then draw that same
number of cards.
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The way it currently works on Gemp is considerably overpowered.
For example, just yesterday I had
Phial of Galadriel on Frodo at Site 7, giving all of my characters Muster. I was able to discard and draw 1 card 9 separate times, allowing me to get exactly what I needed. I loved doing it! But that's not how the rules say it's supposed to work.