There are a number of factors at play here.
If the glucosamine would be derived from hard sea shells, it would be equivalent to rocks which are not a Kosher concern. However, the crab, lobster and shrimp shell generally used are not nearly as rigid and do contain some flavor. Therefore, glucosamine produced from seafood would have to be considered a non-Kosher product.
However, since it is rendered into pills which are swallowed and not eaten in the manner of normal food, there would be some grounds for leniency. If one had debilitating arthritis, or a condition likely to lead to the same, pills that are proven to treat or prevent this condition could be taken under certain circumstances. Whether or not glucosamine qualifies as a "proven" remedy is questionable.
Regardless, one of the conditions is that there is no available Kosher alternative. Today, there are a number of brands of glucosamine derived from fermentation of wheat or corn, and some even have Kosher certification. Only these products should be consumed. A simple Google search for "glucosamine Kosher" will locate a number of Kosher certified brands available for purchase online. A careful check will reveal that some of them are the more beneficial glucosamine sulfate.