Singing out of time or behind the beat

[Glen Retheash]

I sing with an amateur ukulele group and while we were going OK we had a lady join our group who sings (and plays) behind the beat. Nearly a whole beat late. She has a good strong and loud voice and some members of the group follow her timing rather than sticking to the correct timing. This has thrown the whole group into chaos. Songs we once did pretty good are now awful.  More and more people are singing to her timing. She is taking over the group. Has anyone else had this problem or am I imagining it? I played an instrument in a brass band for years so I think I have pretty good timing. I don't think most members realise what the problem is. Any suggestions? HELP