A dobro capo "sandwiches" the strings between a bar and a pad. They come in a few different styles.
Shubb C6b - This is often the "first" capo for dobro players because it is so similar in form to a traditional guitar capo - in cost as well . One major advantage to this style capo is that the brass bar sits on top of the fret so it is always accurately lined up to play in tune. A considerable disadvantage is that each capo needs to be adjusted and set up for the guitar it is going to be used on. If you're using a Shubb, consult their website to make sure it is properly adjusted for the resonator guitar on which it's being used.
Charlie's Slide Pro Capo - This capo ignores the neck of the guitar and attaches directly to the strings. Charlie's capo also has great one-handed adjustment with the wedge mechanism it uses to tighten the capo to the strings. It is small but heavy - a good thing - to get the most tone out of the instrument. Since it is attached only to the strings, a tuner - or very good ear - is essential to making sure it is positioned in the correct place. Sometimes perspective can cause it to be askew. It is expensive, but is sold directly from the manufacturer so there's never the hunt to find it "in stock" somewhere.
*Not Pictured - Beard Wave Capo - This capo functions in a similar manner to Charlie's. The main difference is that the Beard Wave uses a screw/wheel to tighten the capo to the strings. This capo is a little more refined looking than Charlie's, is slightly more affordable, but can be difficult to find "in stock" at this time.