Denominations are not God-made, but man-made: Denominations come into being when there has been a revival and over the years the revival slowly disappears, the persons who have become Christians during the revival will have been given some name (ie Methodists), which then with the disappearance of the revival becomes the name of the denomination, the members of this denomination, who will have lost all the Presence and Power of God, will, instead of seeking God for His plans and purposes and His Presence and Power to fulfil them, will then form committes etc..
I would go along the lines that if a person believes in the Lord Jesus as Lord and Saviour (Rom 10v8,9,13), and so, who is a member of the Body of Christ is a Christian and so someone I would fellowship with and also, if off the opposite sex would consider as a potential girlfriend->wife.
All sectarianism should be avoided like the plague! 1Cor 3v1-4.
I do not consider Mormons or Jehovah Witnesses to be Christians, but heretical sects.
The above is my take on this.