The issue of church attendance versus fellowship is a twofold issue 1st) regarding affiliation/participation within a given denomination, and 2nd) affiliation/participation within a given congregation.
We will start by defining what is meant by the word Church. Without getting too technical, here is a quick breakdown (I hope with brevity and clarity). In the Bible, the word translated as "church" is ekklēsia = call out. The word is derived from ek and kaleo. Ek = out, kaleo = call. This term, in Jesus' time, was used to describe a meeting or an assembly of people. People were called out to hear something. The group called out together was just a gathering of people - called a church. As Christianity grew they began calling a localized group of believers a church - much as today we call a local group a congregation. All believers, worldwide, form "The Church" the "body of Christ". If you are a follower of Jesus you are a member of "The Church" regardless of denomination or whether you fellowship with a local congregation or not. This membership, in "The Church", is mandatory for salvation; and, no other membership is required.
But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanses us from all sin.
(1 John 1:7)
Next, we'll look at membership in a church in light of how it is more commonly viewed today within a given denomination or congregation. Denominations are a modern version of the Jewish sects of Jesus' time i.e. Sadducees, Essenes, Pharisees, and etc. The wrong affiliation could hinder or hurt your growth as a believer.
Minor doctrinal differences though are not the same as major ones. Jesus warned us:
In the meantime, when there were gathered together an innumerable multitude of people, so much so that they trod on each other, he began to say to his disciples first of all, Beware of the leaven of the Pharisees, which is hypocrisy.
Although we may have differences of opinions we have a need within us, as social beings, even without a Biblical imperative directing us, to associate with others. It would be great to be a member of a congregation where we could say:
And all that believed were together and had all things common.
However, today, that would just about mean living in a very small village or a remote commune. That's not necessary because we know Jesus said:
For where two or three are gathered together in my name there am I in the midst of them.
Wherever you live you can try to do the following:
And they continued steadfastly in the apostles' doctrine and fellowship and in breaking of bread and in prayers.
Each person must decide for themselves if membership in a particular local congregation is for them. I would suggest two primary considerations, both being guided by the Holy Spirit. 1st) do you believe that it can and will allow you to utilize your gifts to serve Christ, and 2nd) that it can fulfill your need for sound Christian growth and fellowship.
Any denomination or congregation that is Christian will let you come and worship without membership because they know worship is more important than membership. Anyone who has read my article "Concerning Sabbath Keeping" knows my thoughts on the difference between the Sabbath and the Lord's Day. That said it will never stop me from participation in attending a Sunday worship service or Sunday school service when I can get there.
I personally have fellowship, via the web, with countless fellow Christians. I have found I can best use my gifts in this manner; but, I still look forward to the less autonomous fellowship I get from participating in activities with others at the Old Town Mission or another food bank where I volunteer. Such fellowship is invaluable to me.
I have known those who don't belong to any local church who are a tremendous blessing with their service to Christ a stranger would never guess they don't go to a local church. I have also known those who are members of, and regularly attend a local church, which do nothing else a stranger would never guess they were Christians.
Then there are those who can't attend regularly or are greatly hampered the invalid, the shut in, hospice residents, prisoners, the poor and homeless. That is one of the reasons local churches should have outreach programs i.e. for widows, the homeless, and local jails. Many have expensive programs to send missionaries overseas and don't serve their own local communities.
Still, consider that membership in a given congregation may allow for better administration of the gifts that are available there. It also provides the average unbeliever a place to turn to if the Spirit moves them. We are all a part of "the body of Christ" and as such we need to work together. Just remember what Jesus said in Revelation to the various churches. If you feel your church is lukewarm don't just up and go elsewhere try to heat it up first. If you feel your church is sinking don't abandon the ship till you try to plug the leak and have bailed out the water for a while. Most ministers would revel in the help and God would love it too! If you are ostracized for these efforts then you need to rely on the guidance of the Spirit to see if what needs changed is you or your place of fellowship.
Beloved do not believe every spirit; but, test the spirits, whether they are from God - for many false prophets have gone forth in the world.
(1 John 4:1)
Because, by their fruits you shall know them.