What instruments is among the big five of bluegrass instruments?

Bluegrass band instrumentation varies depending on who is available and what they want to do for each song. However, some instruments are associated with Traditional bluegrass.
1) The Banjo is a mainstay of bluegrass music, so much so that the sound of one being played with frailing or rolling technique makes anything sound bluegrass (even when it isn't intended to!) The bluegrass banjo of choice is a five-string model with metal rim, snare-drum head and tensioners and the fifth string shorter than the others (so that most of the time it is un-fingered and played as a drone.) The banjo can provide a fundamental arpeggiated harmony or play solos, and the different ways it can be tuned may allow whole runs to be played without moving the left-hand fingers a lot!

2) The guitar is as necessary to bluegrass as anything else, the most common type being the flat-top acoustic played with a flat pick, either in 'rhythm guitar' chords or fingerpicking style (the most common being "Travis picking", where the thumb alternates between bass and tenor notes and the other fingers contribute chordal notes in between). The guitar can also take solos, using the flat pick in alternate directions to sound each successive note. Some other varieties of guitar (almost always 'flat-top' acoustic sorts) have become embedded in Bluegrass music, most notably the Dobro and the National guitar. The Dobro is basically an acoustic guitar played with a slide, but with the strings lifted and held 'flat' to make it easier to do slide work on. Some Dobro players are quite good at mixing the slide with fingered notes. The National guitar features a metal top with a cone-shaped addition behind openings in the top. Although most call this an amplifier, it is actually an impedance-matcher, coupling the sound more strongly to the air. Both Dobro and National Guitar can be played with slides and lifted strings or like a normal acoustic.

3) The Fiddle is also a characteristically bluegrass instrument, although technically there need be no difference between a 'fiddle' and a violin other than, as the saying goes, attitude! Bluegrass fiddle styles vary and can sound anywhere between Irish fiddle, classical violin, Cajun romp, or any other variety, as the player feels will fit in well with the music. Slides and chords, train-whistle sounds, whatever carries the emotion of the song usually falls on the fiddle because the other instruments are plucked.

4) The mandolin (tuned like the fiddle, but with doubled strings, frets, and played with a flat pick) is used for solo work, often in sixths or thirds with the fiddle, or in playing punctuating off-beat chords.

5) Finally, of the 'big five', would have to be the string bass. Like the fiddle, there need be no difference between an orchestral bass and one used in bluegrass, other than how it is played (and even then, any orchestral techniques are fair game for bluegrass!)

Out side of these instruments, some bluegrass bands add a drum set (often called 'trap'), keyboard, or other instruments (even pedal steel guitar, although that tends to move the band out of the pure 'bluegrass' category!) And, of course, the most important instrument in a bluegrass band is the voice, because without, you'd lose all the songs.