If you need the plural form, you do NOT add an apostrophe, simply an "s". Thus, Mr & Mrs. Sullivan are "the Sullivans".
If you need to make that plural form possessive, that is, to speak about something that belongs to the Sullivans, you add an apostrophe AFTER the "s" of the plural. So,you would say "The meeting is at the Sullivans' house" or, more briefly, "The meeting [or just "It"[ is at the Sullivans".
The form "Sullivan's" is the SINGULAR + possessive (formed with apostrophe-s). Thus, you would say "This is John Sullivan's book." It is very unlikely you would use the expression "the Sullivan's" in this case. (In other words, there is little or no use for "the Sullivan's".)