Having your friends help out with wedding details can definitely save you money, but it's important to make sure that both you and your friends know what your expectations are and that you know what their limitations are.
A friend of mine offered to do her friend's hair for her wedding this weekend and after I asked a few questions yesterday we discovered that the friend was not told what time she needed to be at the hotel, the wedding is a few hours away from where she lives, and there was some confusion as to whether the bride was doing part of the 'do or the friend was doing all of it.
I told my friend she need to call the bride and find out when she was getting ready because the bride can't put her dress on until her hair and makeup are done and the bride may plan on the photographer (another friend) to take pictures while she is getting ready.
While friends may offer their skills as a wedding present (saving them money) and you may take them up on it (saving you money) it's important to know exactly who will do what and when. In the situation of a photographer who is a friend you may expect that he or she will take pictures all day and night and supply you with a wedding album, while your friend may plan on taking pictures but not making you an album for free.
It's important to talk and not necessarily get everything in writing, but make sure that there's a complete understanding between both of you as to what you expect and what they are really capable of doing. Just because your groom's college buddy owns a video camera doesn't mean he can be your videographer and just because your childhood friend likes to bake doesn't mean she can make you that five-tiered cake you found in a wedding magazine.
So if a friend or relative does offer their services to you as a wedding gift make sure that you really like what they do and that they can do what you want. It's okay to say no. Just explain that you've already hired a vendor for that part of the wedding. They'll understand.