I mostly play "unplugged", whether it be a jam party or a house concert or just porch picking. Years ago, I had a trio which performed sans amplification at coffeehouse. Of course, most of this experience was with guitar and voice (only been playing uke a year now), but I'll share what I know.
Sing clearly and keep your chin up, projecting out to the back of the room.
Rhythm is everything. A strong beat propels your vocal and gives it a foundation, and the vocal is what people will be listening to.
Relax, and be conservative with your playing. Solos and other areas of refined technique are lost in these situations. House concerts are a major exception in this regard.
Familiar songs are easier to "hear". Play songs people like, songs you sing well, and with few or no "deep album tracks".
Uke specific: I bring a Kanile'a to a regular picking party with fifteen or more players including congas, trumpet, accordian, upright piano, dobro, banjo, fiddle, numerous guitars, singing, background noise, etc. It is most certainly a loud instrument but it wants a plectrum to cut through with clarity, so I use one there.
Playing acoustic is a special sound which I really enjoy. Have fun! I'm sure you will make many folks happy through your performances.
