I believe the context is clear about fellowship. First, He begins the chapter on the subject of producing fruit, which is about the believer's lifestyle. Then, He notes that the remaining 11 disciples were saved, from v.3:Can you show the specific Scripture reference where this is recorded or are you deducing this from the context?
"You are already clean because of the word I have spoken to you." Back in ch 13, when washing the disciples' feet, He used the word "clean" to indicate that "all of you are clean, but not every one (meaning Judas).
Then in v.4 He says: "Remain in me, and I will remain in you. No branch can bear fruit by itself; it must remain in the vine. Neither can you bear fruit unless you remain in me."
If this command (remain in Me) is about being or staying saved, then one's salvation is dependent upon oneself, not Christ. We do not and cannot save ourselves. So the command cannot be a reference to either getting or staying saved. It's all about fruit production, not getting or staying saved. The RCC believes that the believer co-operates with Christ in their salvation by the term "synergism".
And, the wording indicates a partnership between the believer and Jesus. The branch (believer) cannot be independent of the vine and bear fruit. By Jesus adding to the command to "remain in Me" that He will "remain in you" shows the partnership.
The last sentence of v.4 also shows partnership for the purpose of bearing fruit rather than getting or staying saved. iow, the believer must "remain in Him to bear fruit".
The concept of fellowship is found in Phil 1:5 - For your fellowship in the gospel from the first day until now KJV
The word for "fellowship" is koinōnia:
1) fellowship, association, community, communion, joint participation, intercourse
1a) the share which one has in anything, participation
1b) intercourse, fellowship, intimacy
1b1) the right hand as a sign and pledge of fellowship (in fulfilling the apostolic office)
1c) a gift jointly contributed, a collection, a contribution, as exhibiting an embodiment and proof of fellowship
Of the 20 occurrences in the NT, the word is translated as "fellowship" in the KJV 12 times. 4 times as communion, and once each for "communicate", "communication", "contribution" and "distribution".
The 4 times we find "fellowship" in 1 Jn 1 it is this word 'koinonia'.
One would have to argue that one is saved by bearing fruit to use John 15 as support for loss of salvation doctrine. How does that square with Scripture on how to be saved? It doesn't.