It is true that glucosamine is mostly made from shellfish (although sometimes also shark) and it is often not recommended for people with shellfish allergy.
Most people with shellfish allergy however are allergic to the protein in shellfish and glucosamine is made from a carbohydrate called chitin. This means that in theory those people shouldn't have an allergic reaction.
I know of one test with 6 subjects, that were allergic to shellfish, that didn't have any side effects from taking glucosamine. More thorough testing should prolly be in place though.
I'm not personally recommending you to take it though, since most labels says not to.
If you want documentation on this try google with "glucosamine allergy" as keywords. Lots of pages should pop up.