A neck capo at fret 2 raises the four neck strings by 2 semitones and leaves the short fifth string exactly where it was. That asymmetry is the whole subject: on a guitar a capo is one action, and on a five-string banjo it is two.
What it gives you
The open strings spell G with no capo, so at fret 2 the same system sounds in A. Nothing about your left hand changes — the shapes are the shapes, and only their names move:
G → A, C → D, D → E, Em → F#m, Am → Bm
The fifth string
The fifth string's nut sits at fret 5, not at the nut proper — the string is short and starts partway up the neck. So to raise it by the same 2 semitones, the spike or fifth-string capo goes at fret 7, which is 5 further along than the neck capo. The drone then sounds A instead of G.
| String | Open | Capoed |
|---|---|---|
| 4 | D | E |
| 3 | G | A |
| 2 | B | C# |
| 1 | D | E |
Doing it
Capo the neck first, then the fifth string, then retune — clamping a banjo neck almost always pulls something sharp, and a drone that is a few cents out is audible immediately because it never stops sounding. Check with the tuner before you play a note.
Your rolls are unaffected: a capo transposes the whole system and leaves the right hand identical, which is exactly the argument for capoing rather than retuning when all you want is a different key.