In a previous recent song of the week email ('I Can't Feel At Home In This World Anymore'), I discussed the '2' chord at length - if you would like that email to be sent again to you, just ask me and I will resend it to you - but there is something more that I would like to add to that, and it concerns the use of dominant 7th chords in place of major chords.
Just as one may substitute a dominant 7th chord (usually called just simply a 7th chord) in place of a major chord for a '5' chord (e.g., D7 in place of D when playing in the key of G), so the same is also true for '2' chords (e.g., B7 in place of B when playing in the key of A)
Notice on the Mac Wiseman live performance included here, Mac is playing a B7 instead of a B: and this is common practice for bluegrass rhythm guitar when a B chord shows up when playing in any key without a capo in which the B chord functions as a '5' chord (key of E), a '6' chord (key of D), a '3' chord (key of G), or, in this case, as a '2' chord (key of A).
However, B7 in place of B would not work well if the B chord were functioning as the '1' chord (key of B), except when used as a transitional chord to lead from the '1' to the '4' (in the key of B, one might for instance play the first half of the I'll Fly Away progression as: BBBB7EEBB), and would not always work well for the '4' chord either (key of F#), and would almost never work for the b7 chord (key of C#). Most bluegrass rhythm guitar players need not concern themselves with this since most of them would never consider playing in any of these keys without a capo, and the two latter keys are not among the 8 Major keys that bluegrass songs are commonly played in at jams. But, it is good for all to be aware - regardless of which instruments they play - that there are only certain chords for which it is 'safe' to habitually substitute dominant 7ths in place of a majors.
When playing in the key of G in standard G tuning, banjo players may often automatically play a dominant 7th in place of a major for the 2 chord (in the key of G, an A7 chord in place of an A chord) without being consciously aware that they are doing so, for the 5th string - the short string - on the banjo is tuned - when in G tuning and when not capoed - to a G note (banjo players rarely ever fret this string), and this is the very note that when added to an A chord makes it into an A7 chord. (This same A7 chord will also often show up in place of an A for the '6' chord when banjo players are playing in C without a capo.)
To make any major chord a dominant 7th chord, all that one does is add to the chord the note that is a whole step lower than the note that has the same letter name as the chord: this is the b7 note/scale degree on the Nashville Number System Chart handout.
I think it sounds best if only some of the players at any given time, rather than all at the same time, in a band, or at a jam, use the dominant 7th in place of the major when playing over '2', '3' and '6' chords. On '5' chords, I like to hear the dominant 7th used even more sparingly.