Question:
Is it wrong to be involved with believers outside the fellowship in helping with evangelization, etc.

Answer:
It is certainly not wrong to “Go…into all the world, and preach the gospel to every creature” (Mark 16:15), for this is the Lord’s great commission to every believer. It is not wrong to work together with other believers in getting the gospel out praying together and to pass out the tracts and invitations to a meeting where the gospel of God’s grace through Christ to lost sinners is clearly preached.

God has called every believer with the “high…holy…and heavenly calling” (Philippians 3:14; 2 Timothy 1:9; Hebrews 3:1). A calling whereby “ye are called in one body” (Colossians 3:15) to work together as directed by the Head of the body, even Christ.

The Lord desires to direct us as members of “the one body.” Any gift He has given to us is for the good of the whole body of Christ: “For the perfecting of the saints, in view of the work of the ministry, in view of the edifying of the body of Christ” (Ephesians 4:12, JND translation).
God’s plan is that the evangelist should not only draw souls to Christ for salvation, but that he should then introduce them to the assembly (the “two or three gathered unto His name” Matthew 18:20) where God has provided the pastors, teachers, and other gifts to care for them and to cause them to grow in the Lord’s things. Salvation is only the beginning of what God has for the believer, and He has given sufficient gifts in every assembly for the blessing of souls (1 Corinthians 1:7; Matthew 18:20). What a privilege it is to function in the assembly, seeking to be led of the Lord, showing forth “the one body,” for the edifying of all.
One problem some have found in working with believers outside the assembly in evangelization comes after souls are saved. What church is the convert going to be directed to, the assembly or the church of the believers you are working with? Another problem many find if they try to work with believers outside the fellowship is that they are told how to do things. It would not be right to be under the direction of anyone, or any group of believers (other than the Lord), in our service for Him. The assembly has no authority to direct the Lord’s servants (the Lord does through His Word), and each one of us answer only to the Lord Himself as to what we do. “To his own Master he standeth or falleth… Everyone of us shall give account of himself to God” (Romans 14:4, 12).We thank the Lord for wherever (and by whomever) the gospel is preached, even if it is “of envy and strife” (Philippians 1:15, 16). But if we are going to please the Lord, we must follow the direction He gives us in the Word, which has in view the function of His assembly in connection with His work.