Visual programming has traditionally supported only one person working on a program at a time. Researchers in the field of computer supported cooperative work have developed text and graphics editors that allow multiple people to work on the same document at the same time from multiple computers viewing the same, shared document. An unresolved problem is how to avoid interference between the users. A technique based on domain syntax is presented here. It is shown to be effective in preventing such interference during collaborative development of Entity-Relationship diagrams using CoDiagram, a proof of concept system. Twenty groups of thee people developed diagrams and rated the system as usable.