Cite their work in future papers you send out for publication, if appropriate, just as you would with any other relevant work you're aware of at the time you send out the manuscript (incl. any revised manuscripts).
As others have suggested, consider sending them a copy of your thesis, which they could cite in their own future publications. This would be especially helpful to the advancement of the science if you've made advances on parts of the problem they might have been getting stuck on, or if your independent works constitute independent validations of the same observed principles.
Don't worry about the past publications when you weren't aware of the other group's work. It's not reasonable to expect every researcher to be aware of every unpublished working paper even if directly on topic and even if previously presented.
Independent simultaneous invention/discovery is common, because multiple groups have access to the same memetic/conceptual building blocks and tools at approximately the same time. See some of Steven Johnson's work, like Where Good Ideas Come From (trailer here), for more on this.
Even if it were previously published work that many people in your field were familiar with, the most serious likely impact would be earning an (in that case appropriate) reputation for not doing sufficiently thorough literature review and/or not being sufficiently well connected as part of the research community on this topic. Such a reputation might make funders a bit more hesitant to give you grant money (if they think it's more likely to go to reinventing the wheel than granting the same resources to someone else) but it's neither reasonably expected nor efficient to have you and every other researcher aware of every unpublished working paper with ideas being discussed among colleagues for possible improvement.
See also linked "related" questions on the right.