How do we ensure accountability when students are working in groups?

Students may get a friend in the same to cover for them. How do you control that?
Use of peer & self-assessment worksheets will help students realize their accountability.
What if two students in the group do not like each other?
Do we always like people we have to work with? Real - life situation they can learn from now.
Do we want to choose groups, leave it to chance, or let students pick?
Use a variety of methods to group students. I usually start with students choosing their own partners, and then group partners to create a team.
How do we choose group leaders?
I allow the teams to brainstorm & discuss the project informally to see if a leader emerges before asking them the chose a leader. It is more successful if the team chooses their own leader.
What if someone in the group does not complete their work, or does not put in the same amount of effort as other members of the group?
Again... the use of peer & self assessment sheets will help the instructor to gain information about the efforts put forth from each student. The peer-assessment gives students the chance to discuss problems issues, while the self-assessment gives the student the chance to show the knowledge they gained.

I think that the concerns listed have been around since we have assigned students book reports. I can still remember being in Sr english and being the only one in the group to read the book.

Very valid concerns! I used to hate working in groups b/c I was always the one who did the most work. AND no one ever taught me how to deal with "slackers"...I was afraid they wouldn't like me if I made them work!