I plan on shooting groups of 15-30 people. What poses can you recommend for shooting a social organization? Is a 35-70 lens on a Canon T50 suitable for shooting groups of this size? In the past I've used a 28mm and the individuals on the end look out of focus.?

I would not use a 28mm lense or a zoom lense on a large groups. If distance is the problem, try shooting your groups outdoors on an overcast day or under shade. However, try using a medium length lense like a standard 50mm that usually comes with the camera. If you chose to buy the 50-70mm zoom instead, use that. Hope it helps.