Both Uses of

muster
in
Paradise Lost

Part wield their arms, part curb the foaming steed,
Single or in array of battle ranged
Both horse and foot, nor idly mustering stood;†
Book 11 *

Definitions:

(1) (muster as in: muster strength or the crew)
to gather
The exact meaning of this sense of muster is often subject to its context. For example:
- "mustered her courage/strength/a smile" -- to gather from within
- "muster the soldiers/crew" -- to gather -- typically for inspection
- "muster volunteers/votes" -- to gather enough to do something
(2) (muster as in: didn't pass muster)
inspection -- especially of soldiers or a crew
(3) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

More rarely, muster can refer to a counting or census. Much more rarely, muster can refer to a call to duty such as compulsory military service or jury duty. In that sense, you may see the expression muster in or muster out. Still more rarely, a muster can refer to a group of peacocks.