I read brother Hamilton's article on Cross Congregational Fellowship with keen interest and have this question: Can fellowship be withdrawn from members who have absented themselves from their local congregation for months, but who have gone to another congregation to worship without first advising their local congregation that they will be worshipping elsewhere?
Anyone not attending a congregation cannot be considered a member of that congregation. "Let us hold fast the confession of our hope without wavering, for He who promised is faithful. And let us consider one another in order to stir up love and good works, not forsaking the assembling of ourselves together, as is the manner of some, but exhorting one another, and so much the more as you see the Day approaching" (Hebrews 10:23-25). It is not that they are not Christians or are in sin. It is not a sin to attend another congregation.
Withdrawing is to remove contact from someone who is in sin, in part to prevent the spread of that sin to other members of the church (I Corinthians 5:6-7). Yet, in this case, no sin is taking place. It might be called rude to change congregations and not let others know about your decision, but that would be the extent of it.
A simple letter stating that since they haven't joined the saints in worship and work for several months and that you have heard that they are now attending another congregation, the letter is acknowledging that they are no longer considered members at this congregation. The letter is not required, but it sometimes helps to make things clear.