If the surname is Mullin (no s), you would add an apostrophe s, as in the Mullin's house (a house owned by the Mullin family.) Thus, "I went to the Mullin's."
If the surname is Mullins (with an s), you would add just an apostrophe. ( I think.) I'm not 100% sure. This is a question I've had myself. It would either be, "I went to the Mullins'." or "I went to the Mulli
After initially disagreeing with Mr. Wordy, I did some further research. I believe what Mr. Wordy is giving you is the plural forms of the surname(s). However, I'm not convinced that those are appropriate in the context of this particular sentence.
I went to visit the Mullins. / I went to visit the Mullinses.
The plural forms are appropriate here, because these sentence