Collaboration

Although most esolangs are created by a single author, sometimes, multiple authors can work on a single esolang. There are many ways people can collaborate.

User-edited pages

This is the most common type of collaboration. Someone makes a page on the Esolang Wiki (or the Esolanging Wiki), and then declares that either 'anyone can edit it' or that a specific set of editors can edit it, usually with some rules that an editor may follow. Some examples include BFInfinity, UserEdited, Usernames, ™, CAPI, and User talk:User:Main page/w/wiki/User:NotPrySigneToFry/what. Most user-edited languages fall under the first category, while some such as CAPI and Lingua Indeterminatum fall under the second category.

via instant communation

Some people that already know each other might use instant communication methods like e-mail or QQ (china mainland) to communicate to each other regarding the matter of esolang creation. One example of this is robots.txt.

Version control

Some esolangs but mostly esolang implementations are developed collaboratively using version control systems like Git or Subversion, usually hosted on forges. The Esoteric Combine project and cfunge are examples of such implementations of esolangs, while 🏳️‍⚧️🦞 and RSI0 are examples of such esolangs themselves. This approach for developing esolangs is a lot rarer than the others.