There's no definitive "canon" answer about HOW Iggwilv brought Graz'zt to the Material Plane (he was in the middle of a fight with harpies and troglodytes apparently), but several books use the specific language that she summoned him.
I know that somewhere in 3e there specifically was a "Summon Graz'zt" 9th spell that was in Iggwilv's Demonomicon or one of the various spellbooks she made (a couple such books appeared in Dragon magazine), but I'll have to double check which source that was from.
When it comes to True Names, 5e really hasn't explored that mechanically (at least not yet). The Monster Manual says: "A mortal who learn's a demon's true name can use powerful summoning magic to call the demon from the Abyss and exercise some measure of control over it."
There's also a lot of ways you can choose to spin that story. For example, troglodytes are associated with Demogorgon (one of Graz'zt nemeses), so maybe Iggwilv used gate and lured Graz'zt through / offered him an escape route. And then he ended up in an inverted magic circle she had prepared, and Iggwilv cast planar binding on him. That would be one possbility.
Another possible way to impose long-term control over a demon lord would be to acquire its demonic amulet (see Monster Manual page 51). "A creature possessing a demonic amulet can exact favors from the demon whose life essence the amulet holds or inflict great pain if the fiend resists. If an amulet is destroyed, the demon that created it is trapped in the Abyss for a year and a day."