Jesus calls out to a lost world, "Come unto Me, all ye that labour and are heavy laden, and I will give you rest" (Matt. 11:28). In other words, Jesus is saying, "Quit using the works of religion to try to be worthy of a right-standing with God; in Me, you will find rest from works." Those who respond to the Father's drawing to a saving trust in Jesus, become born again believers (Jn. 3:3-8; 6:44). Salvation is a free gift (Eph. 2:8-9); works can never achieve salvation (Gal. 2:16).
Once a person places their trust in Jesus, he becomes a part of the Church, in the Greek ἐκκλησία (ekklēsia) meaning "a called out assembly to be in the world but not of it" (Jn 17:9-23). There is another Greek word which appears once as "assembly," συναγωγὴν (sunagōgēn), which means "a coming together" (Jam. 2:2). It is translated "synagogue" fifty-five times elsewhere in the New Testament. James was writing to "the twelve tribes which are scattered abroad" (Jam. 1:1), so perhaps he used the common word the Jewish believers would better understand.
The Church is to assemble for the purpose of worship and praise; testimonies and thanksgiving; Baptism and the Lord's Supper; instruction in the Word; reproof and correction; doctrine and instruction in righteousness (Acts 2:41; 1 Cor. 11:18-26; Eph. 4:11-13; 2 Tim. 3:16; Heb. 10:24-25). The Church is to go out to share the Gospel with the lost; to minister to the hurting, the hungry, the naked, the homeless, the widows, the orphans, and to those in prison (Matt. 25:31-40; 28:19-20; Jam. 1:27; 2:15-17).
My pastor calls Church meetings "refueling" for the work of the ministry. Jesus wants us to come together, but He also wants us to go out with the Gospel and to serve. We don't become Christians by works, but if we have accepted the free gift of salvation, we will want to do His will out of gratitude.
Born again believers come and go as their Lord pleases!