As always, everything in moderation. A small bowl or cup of tea won't do any harm, but too much caffeine is dangerous for dogs. Mine usually take it in turns to lick out the bottom of my cup. I would not put any sugar in it though, other than any that you already have in your own tea.
Many people give their dogs tea when they have theirs, they don't really need it, but they feel like they are sharing (and the dogs sit and stare at them) and as I said, a little won't do any harm.