A 7th chord is a 4 note chord built upon a triad (3 note chord). In bluegrass we primarily use major and minor triads.
Occasionally a dominant 7th chord will pop up to build tension for the next chord change, but 7th chords are way more popular in show tunes and jazz.
I've separated the 7th chords into two groups:
1. Major Base - Take your root position G chord, and lower the pinky note either one (maj7) or two frets (dominant 7th). For the major 7th chord you will need to switch your fingers around to maintain the original triad pitches.
2. Minor Base - Start out with a minor triad chord and repeat the process you did for the major base.
Do this in several different keys. Root position 7th chords are only the tip of the iceberg. We still have 1st, 2nd, and 3rd inversions for each 7th chord! Those lessons will be coming soon :).