Morningstar Farms and Worthington make a veggie sausage link along with many other meatless products. You should be able to find them in the frozen, or health food section of your local super market. That is of course depending upon where you're located. WholeFoods would be a good source if you have one in your area. Otherwise, google 'em up.
I've tried them both on pizza in the past and they're not half bad. That said, having been vegetarian for close to 35 years, it may be a safer bet to go with just veggies, for your veggie oriented friends. We're known for being a some what, persnickety bunch.
Below, a couple shots taken before my camera went on the fritz. I did up 2 of these for my carnivorous friends and family. Sadly for me, there were no leftovers to munch the next day.
Red onion, red bell pepper, green olives, mushrooms, artichoke hearts and feta cheese... topped with mozzarella and gruyere (can't find provolone around here anywhere). Forgot the fresh basil this time round.
Good luck and enjoy!