User blog comment:Jackalex13/Is it a glitch or somethin?/@comment-4808313-20120206172500

I've found that on some quests for the Companions that your quest-giver, or whoever is tagging along with you for the quest, will 'run ahead and meet you at the cave/camp/whathaveyou'... so they aren't following you around and are actually standing outside of the quest-location waiting for you... but are still considered your 'follower' and this keeps you from recruiting someone else until you finish that quest.

Might not be your situation, but I thought I'd mention it just in case.