This chapter gives us an interesting picture of how the influence and power of David rose from a little grumbling into a mighty crescendo of unity in Israel. It started with David's departure from the court of Saul. As he hid for his life, he was joined at first by a group of 400 men who one might think (from 1st Samuel 22:2) was only the disgruntled misfits, unfortunate debtors and worthless ne'er-do-wells from all over Israel. However, this chapter clarifies things somewhat.
First, this chapter mentions a group of about 2 dozen ambidextrous Benjamites who severed their familial ties with Saul (who was from their own tribe) to serve David. Then there were 11 elite fighting captains from the tribe of Gad who came to follow David. 1st Chronicles 12:8 tells us that they were fierce and agile warriors. Following these were a band of captains out of Benjamin and Judah who hailed under the banner of one, Amasai. Thereafter David gained seven mighty men from the tribe of Manasseh along with them came the loyalty of the thousands of soldiers who were under their command. And so, in this way David's force grew slowly at first and then more rapidly until he had a massive following of elite military strength (1st Chronicles 12:22).
So, at his inauguration, David's ceremony was attended by well over 300,000 men in military uniform (many of them leaders of larger groups of men who couldn't or just didn't come to the big 3-day hoo-hah – 1st Chronicles 12:40). And, why shouldn't it have been? Israel finally had an able king who loved God and who loved his people. It was a glorious time in Israel's developing history.