This is actually possible! Just create a normal couple (i. e. the celebrity and their spouse) and then click on the celebrity (if you want the celebrity to be the person with multiple partners) and choose the button "Add partner/ex" on the sidebar. This gives you the option to either add "new primary partner" (which turns the current spouse to an ex), "new ex-partner" (which adds the new person as an ex partner) or "new extra-partner" (which creates a new, current partner, so that the celebrity has two current partners).
If you want to display a polycule (not "celebrity + partner a" and "celebrity + partner b" but a threesome/foursome/fivesome/whatever in which all people are in a relationship with each other): After adding the second partner to the celebrity, click on the original spouse, again choose "Add partner/ex" but then the option "Add partner already on the tree", choose the recently added partner and then "Add as extra partner". In theory, you could do this as often as you'd like, though the tree tends to get a bit messy when one person has too many partners.
Hope this helps!
