Beginners are welcome, especially during the first 30 minutes when basic chords and strums are taught and then incorporated into familiar tunes. After that, it gets faster and the music gets a little more complex but everyone is welcome to try their hand at the tunes and there are shared books of music to use so it's not completely by ear.
But be prepared! There's a lot of singing at the Ukulele Jam so you'd better brush up on the lyrics to Maxwell's Silver Hammer, You Are My Sunshine and (yes!) Pinball Wizard.