First, I would say that we take an incorrect view of the word "fellowship". We've come to define the term simply as "hanging out together", usually with some kind of food or fun or both, like a quasi-religious birthday or high-school graduation party.
This isn't really the Biblical meaning of the word. In fact, last as I recall, in every instance, the Greek word for fellowship and for communion are the same word, translated two different ways. If I'm spelling it correctly, I believe it's
koinonia and its derivatives.
So, one can hang out with non-Apostolics and share a meal and spend time together, but does that mean they are spiritually communing the way the Bible describes it?
Conversely, just because some Apostolics get together for some grub somewhere and chit-chat, does that mean they are truly, Scripturally fellowshipping?
Communion and fellowship in the Bible is an intentionally, deep, spiritual gathering for the purposes of breaking the communal bread of Christ's body and partaking of the cup of God's new covenant in Christ's blood.
It's not just some attempted re-enactment of the Last Supper, but it is a love feast, or feast of charity, and there are some strict guidelines on how those are to be conducted. Jude is a good place to look. So is
1 Corinthians 5.
So, my point is, questions become flawed when they are not based in the Biblical reality of the terminology.
So, unless you are actually participating in a love feast with some Baptists, Methodists, Campbellites, and etc., under the Scriptural guidelines of what that means, it's not fellowship in the truly strict sense of the word. It's just hanging out together. The same is true with or among Apostolics.
And here is proof: We are shown in the Scriptures that we have fellowship/communion with God and His Son through the Holy Spirit.
Now, this being the case, ask yourself: Do you and Jesus "hang out", like a couple of guys sitting on a couch with some chips and sodas watching the NBA finals? I doubt it. Every act of communion with the Lord is a spiritual one, that brings about personal change and devotion to God. The same is true, or at least, is supposed to be true, of fellowship. And singing "happy birthday" to a niece or nephew or playing video-games, even with a brother in the Lord (merely hanging out) does not accomplish this.