When baking, one wrong ingredient, or the amount of the ingredient, can affect the outcome of an entire loaf of bread. The same is true with a worship team. One weak musician (instrumentalist or vocalist) can affect the musicianship of the entire group.
Therefore, it is important that you maintain a minimum level of musicianship among your team. You must set the bar of your expectation, then work to maintain and grow that level of musicianship.
Vocals. All vocals should be able to match pitch. If reading music is important, then be sure they are all able to read music. If singing harmonies is important, then make sure they are able to do so. If range is important (alto, tenor, etc.), then know what range you expect.
Instrumentalists. All instrumentalists should be able to play in tune. They should be able to play with the band. There must be give and take and working together.
Don’t be too eager to build a large worship team. It is better to keep your musicianship strong with a small team than to allow a weak musician to be a part of the team. Set your standards high, then look for those who are able to bring excellence to your team.