Use the following trick to get a three person team with a five person squad. Have your main character ask Brelyan Maryon to join you (after doing her side quest). Give her the staff of conjure flame Atronach. Next, do the "Taste Of Death" quest and convince Verulus to follow. You should now have a total of three characters. When in combat, summon your familiar and have Maryon summon hers. There are now five of you in battle. This makes the game easier when doing almost all side quests. In some side quests other characters will also join you, increasing your squad.

Note: If you complete Mercer Frey's "Scoundrel's Folly" quest, he asks you to join him in another quest. When you accept it you will now have four members in your team. You can travel to do quests with all four members, and when in battle your squad increases to seven with conjuration abilities.