On the other hand just assembling a gun from parts should not constitute manufacture.
Above response is incorrect unless you are assembling for your own use and not for distribution to others.
If FFL holder purchases seperate components (serialed receivers, barrels etc) and assembles parts into a complete weapon for sale (as in this instance an AR) FFL is required to have a manufacturers FFL. if a customer buys the same AR parts independently, (say from Brownells or some other licensed manufacturer), for you to assemble for him you only need a dealer FFL. This is a narrowly defined example specific to the OP AR assembly question posed at start of thread.
I was asked this question recently and called my FFL ATF field agent who confirmed this. When in doubt always confirm with your local FFL ATF field agent contact to clarify.
Specific answer to the OP question posed at the start of this thread is he/she must apply for an FFL manufacturers license. Realize that if you have an FFL dealer license there is no upgrade path to a manufacturers license. The two licenses are independent so if you have a dealer only license, applying for a manufacturers license is a brand new license application that will invalidate the original dealer license once it is issued.
If OP is taking possession of weapon to do work for "friends" without FFL, then OP is not in compliance with federal law.