Here's how to add a person's second marriage, third marriage, etc.
First, go to the person's edit page (click an edit link from their profile page) and look in the right-hand column under "Edit Family." Scroll down to "Spouses" and click "[add/remove a spouse]". This creates the spouse and establishes the relationship.
Note that as an added feature you can opt not to have a previous marriage displayed on the front profile page. After the marriage is created, click "[edit marriage]" on the edit page and check "Do Not Display".
Sometimes you would like a child's step-parent or adoptive parent to display in addition to a biological parent.
Unfortunately, at this time a person can only have one father and one mother.
One thing you may want to do is make a note about the other person in the bio section for the child and/or parent. For example, you might write "After his father's untimely death, Jimmy's mother married Joe Schmoe. Joe adopted Jimmy and raised him as his own son."
If you include a link Jimmy and Joe Schmoe will be connected, but they won't be on the family tree together.