Are there any significant security concerns about adding users to the apache group? I am a programmer and I need access to all the files/directories in httpdocs - is there anything wrong with adding my user to that group?
If you add apache user to your group - this is bad.
If you add you to apache group - where is a problem ?
In your situation you do not want to do this as your intentions pose a significant security risk.
The Apache user/group should only have read access to the files in the web root. If your apache user has write access then you are open to having your application exploited and since it is running under a user that has write access, the attacker now has write access to your system.
I assume when you say that you need 'access' you mean that you want to write to the files. If you were going to achieve this by making yourself a member of the apache group, that would not meet your goal because (assuming you're running good permissions to begin with) your apache group does not have write access to the web files. You would also have to allow the group to write to the files in order to achieve what you want and then you're open to the type of attack I described in the second paragraph.
You should create a new group (www-admins or something) that has write access to those files and then add yourself to that group instead.