Miss Manners might tell you that it is appropriate to bring flowers as a thank you gift when invited to someone else's house for a meal. A simple bouquet does make a lovely present - but if you are going to bring some for Shabbat, here are some basic facts to consider.
Bouquets of flowers are certainly thoughtful, but putting those flowers into water can also be problematic on Shabbat, as this falls into the catagory of m'la'cha (creative work prohibited on Shabbat) called zorayah, planting. Obviously, cut flowers will not continue to grow, but they will last longer in water and possibly blossom.
If you intend to bring flowers for your Shabbat hosts, they should be brought to their home before Shabbat. Once placed in water before Shabbat, the cut flowers may be placed on the table as a centerpiece and even moved about the house, as long as they remain in the same vase of water (and are not placed in a specific location where they might blossom better).
Perhaps you are wondering why moving the flowers is being mentioned at all. It is because the permissibility to move cut flowers distinguishes them from potted plants.
A potted plant is already planted and growing. It seems obvious that just as one may not put cut flowers into water on Shabbat, one may not water a potted plant--but what could be the problem with moving it?
The answer is sunlight. Basic science has taught even the most horticulturally-challenged among us that a plant needs light to grow. When a person moves a potted plant on Shabbat, he/she directly affects the amount of light the plant receives, and, therefore, its ability to grow.