Your question was:
>>How do you deal with an erring brother that has "withdrawn" himself from the local church?<<
One who willfully sins by forsaking the assembling of the saints is walking disorderly (Heb. 10:25-26; 2 Thess. 3:6). While the scriptures do not specifically describe the deliberate action of a person who is "forsaking the assembling of ourselves together" as "withdrawing himself" from the local church, that is the effect of such conduct.
One who does so is in need of our efforts to restore him through repentance (Lk. 17:3). To help accomplish his repentance, the scriptures instruct us that we should reprove, rebuke and exhort the sinner using the word of God (2 Tim. 4:2; Jas. 5:19-20). If he will allow you are someone else in the church to discuss his sin with him, then by all means use such an opportunity to call him to repentance.
When a Christian rejects our efforts to call him back from sinful conduct, the Bible says that, due to his disorderly conduct (in violation of apostolic truth), we are to withdraw ourselves from him (2 Thess. 3:6; 2:15). That is, the local church should publicly note the person's decision to live in violation of truth, and that every effort to restore the fallen brother has, to this point, be refused. Therefore, the church is taking note of his sin and does not approve of his present course of conduct. Then, each Christian is to cease their social contact with that Christian. These combined efforts - public marking (taking note of) and individual removal of social interaction - are designed to bring the brother to shame over his sin (read 2 Thess. 3:14-15; Rom. 16:17 and 1 Cor. 5:3-5, 9-13 for the scriptural descriptions of these actions).
Some brethren resist these actions by responding: "he has already withdrawn himself from us -- we cannot withdraw from him." This overlooks at least one point. Local church membership is a two-way street. When one wishes to identify with a local church, both he and the local church acknowledge that decision. Likewise, if that same person later decides to become unfaithful, the local church has the right and scriptural obligation to make it clear that it opposes such conduct and that the person so conducting himself will no longer be considered a member of that congregation. He is out of fellowship with God and needs to repent. Thus, the church is protected from the influence of unfaithfulness (1 Cor. 5:6-7).
I hope this is of some use to you. I will be glad to clarify or elaborate if there is anything here that you would like to discuss further.
May the Lord bless each of us as we attempt to do His will in His way, that souls may be strengthened and saved through His gospel.
Sincerely in Christ,
Joe R Price
Mt. Baker church of Christ