I've played ukulele in church, but only as a soloist. I had two mics, one for vocals, one for the ukulele. Definitely needed it. Our worship teams are sort of "the right size" so I haven't pushed to join them. But I might join in group stuff at some point. I've been thinking, as you have, what my role would be. I've had a few thoughts:
1. The ukulele is a string/percussion instrument. Or as James Hill says, "It is a strumming machine". You can add lots of rhythmic things. I often do rasgeados, muted strums, that sort of thing. You can add a lot of nice, light rhythms to a texture which sit nicely with kit percussion. This doesn't suit all worship songs, but should suit most anything upbeat.
2. Tremolo melodies work really nicely. It's tricky at first for most people, but if you practice it regularly, you can get a nice, consistent sound. Some people use a plectrum. I've learned to do it with my index and/or middle finger. 10,000 Reasons, by Matt Redman, has a tremolo mandolin part at 2:44 which should give you an idea of the possibilities in the context of worship music.
3. Higher chords voicings, as everyone has been saying. It's good for your ukulele playing to learn chords up the neck. And many can have an open sort of sound, such as C, voiced 0-0-3-7, or even 0-0-8-10.
A ukulele is a big voice in a small package. You just need to know how to coax it out, and punch it through everything else.
But, I heartily agree with everyone that you're going to need amplification in any but the smallest settings.
Keep us posted as to how you get on!