I am in a similar position to yourself, but in reverse!
After many years in charismatic and pentcostal congregations, I have fairly recntly started attending my local Anglican parish church on a Sunday morning - partly because I value the sacramental spirituality, and partly because I have found myself more welcomed and encouraged to play and active part in the life of the fellowship ('serve' as Louis puts it) than in the assembly of which I have been a 'member' for 3 years, yet always felt an 'outsider' - little more than a pew filler (despite having attended a 'Training to Serve' course and acted as an Usher for almost 2 years). However, my pentecostal church does not permit me to continue as a 'member' there and also hold 'membership' in another church. Yet is 'membership' really a matter of having your name on a piece of paper, or about being actively involved in the church? If the latter, then I am probably more a 'member' of the Anglican church than of the Pentecostal on who have me officially 'on their books'!
As far as I'm concerned, as Louis says, my most important 'membership' is of the Body of Christ, and which particular local manifestation of that body I identify with is largely irrelevant - but to identify and be involved and committed to a local gathering is vital for us to be truly functioning as a part of the Body - pulling our weight, and being supported by others.
As to how you can 'serve', I'd say the starting point is to get to know people, offer them friendship and encouragement, pray for them and the moinistry of the assembly, let them get to know you and recognise your abilities - and let it be known that you are willing to share whatever you have to meet their needs (even if, initially that's something as simple as lending a hand to set out chairs, or washing up the cups after coffee - without waiting to be asked, even!). Then let God direct you into whatever line of ministry is appropriate as the time is right.
Anthony