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To aid you in playing with different groups / singers, carry a transposition table with you to transpose music (on the page or in your head) into whatever key is comfortable for the band / singer.
To create a table, write out on a piece of paper the notes in the C Major scale from C up to B (in semitone increments) and then write each other key beneath, starting at the major and going up in semitones. This is a really easy way to transpose chords in an instant.