you'll find that the kit lens is capable of doing the job too, as long as you have a speedlight to provide light.
if you're just shooting at the wedding as a guest, and not as the PAID photographer, don't bother buying a lens just for that one wedding. if you really want to, just rent one.
anyway, the tamron 17-50 f2.8 is fine for weddings. any lens with focal range with a minimum of 18 and above will work. frankly for the group shots, you'll not be using f2.8 much, so there's the kit lens will work fine. the FLASH is what is really important. make sure you know how to use your flash, don't just point it at the subject and blast away.