Recruiting initial board members differs for Chapters and SIGs.
For SIGs, the process is easier. In fact, SIGs are not required to have a board! Instead, SIGs are encouraged to identify a Leader/Chair or co-leader/co-chair (as many as are needed). This more informal, looser organization seems to be more effective for SIGs. However, If SIGs would like create a board, they are free to hold an election in the same way as a Chapter.
For Chapters, initial board members will likely be comprised of the motivated and energetic individuals whom are organizing the Chapter. However, a Chapter must still hold an election (see Elections- how to run them). ACBS is willing to help run the first election if so desired. But don't be intimidated! All this means is that individuals interested in serving as a board member need to be nominated or self-nominate, and that the list of prospective members will take a vote.