We are glad to accept people who play acoustic string instruments that are not normally thought of as bluegrass instruments. But we feel it’s important to make clear why the instrument is not normally thought of as “belonging” in bluegrass.
In general, the reason is the instrument is too quiet or low-definition in sound to stand out among a group of bluegrass instruments. The player can participate by playing along, but their playing may not be heard by the other musicians, so it’s not the usual type of participation, where interactions are part of the artistry and the fun.
Ukulele or mountain dulcimer, for instance, would be hard to hear over guitars, banjo, bass, etc… unless amplified. Amplification could work, but that would open another challenge, since many jams take place outdoors, and the absence of an outlet could be a problem… Then again, there are good battery-operated, rechargeable amps, which would solve the problem.