They went to the park.
I went with them.
They brought their frisbee.
At least I think it was theirs.
They threw the frisbee to themselves.
Full usage: http://pronoun.is/subject-pronoun/object-pronoun/possessive-determiner/possessive-pronoun/reflexive displays examples of your pronouns.
This is a bit unwieldy. If we have a good guess we'll let you use just the first one or two.