Rumours go that there is still a small, but active community of French royalists. They apparently meet regularly in a church close to the Seine in Paris.
Is this rumour true, if so can I join their meetings and where is it?
There are many distinct and rivalling royalists groupuscules in France so it seems unlikely that there is one answer to this question.
It's not explicitly royalist (or even political at all) but the Fraternity St. Pius X tends to be associated with the most conservative quarters of French society, including some royalists. In France, one of their most well-known centers is Saint-Nicolas-du-Chardonnet. It's not far from the Seine so it might be the source of the rumour you heard.
Otherwise, there is this (you will note that a number of these masses are celebrated following the tridentine rite and some of the clerics and churches involved are, in fact, associated with SSPX).