Ukuleles are not part of the typical bluegrass ensemble. Due to a renewed popularity in the United States, they are becoming more common at bluegrass jams.
A major hurdle for ukes is the ability to be heard in such a large group. The combination of the small size and nylon strings makes projecting the sound quite a challenge. One solution to this is playing a banjolele. It is the same size and tuning as a ukulele but being built on a banjo body means that it is typically louder and projects better than a traditional ukulele. If a banjolele isn't an option, then using larger ukulele sizes like tenor- or concert-sized ukuleles could be helpful.
Left - Stella Banjolele - This banjolele was built in the heyday of banjo ukes in the 1920s. It has been upgraded with Ratio Tune-A-Lele Ukulele Tuners. The original "friction" tuners did not hold the tension of the strings very well. These upgraded tuners are "geared" but have a very small footprint.