Singing with a group is different from singing on your own. You must try to harmonise with the other voices, as not everyone can easily sing in the particular key of a song. It helps if you have had some singing lessons. Before taking up the Ukulele I learnt to sing a cappella, that is, unaccompanied by an instrument, in a small group class. We learnt how to find our vocal range and to sing different parts in three or four-part harmonies. It was great experience for singing in a Ukulele group, but anyone can learn to sing in harmony.
It takes practice to hold your part with other singers, but it helps if you sit or stand next to someone with the same vocal range. Over time and by listening to the other members of the group, you should be able to do this with ease and without damaging your voice by singing out of your range. It is well worth the effort to persevere with harmony singing and it will give your ukulele group a more integrated sound.
A good way to develop the harmonies for a song is to sing them unaccompanied before bringing in the ukuleles. You can then tell if they are working and that everyone is in sync before adding the instruments. This makes for tighter vocals and allows everyone to learn their parts.
I have also noticed that in some larger ukulele groups not everyone plays their instrument when they are singing. This tones down the volume of multiple ukuleles and makes it easier for the singers to hear their harmonies. When you are doing tight harmony singing it takes a lot of concentration and it is probably better if you do not have to think about what you are playing at the same time. Also soloist singers in a group often do not play their ukuleles because they are putting all their effort into the vocals.
A good example of the above technique can be seen in the performances of the Langley Ukulele Ensemble from British Columbia. They perform beautiful vocal harmonies and vary the numbers of those playing their ukuleles. Below is a video of a performance that they did this year in Hawaii.
So don’t worry if you find it difficult to play the ukulele and sing harmonies simultaneously. If there are enough ukulele players in the group, it is not detrimental that you do not play your instrument while you are singing and this can improve the overall sound.