Joined for duty and enrolled is a term used on muster rolls and usually precede enlistment dates. As you will note, these fellows were enrolled by the same individual. G. D. Alexander, the mustering officer for Co. F, 26th Arkansas.

Often, on muster rolls for men who've transferred from one unit to another, the enlistment dates shown on the latter rolls are the dates of enlistment for the original unit.

Algenon B. Barnes, Private, Company F, 26th Arkansas Infantry, enlisted June 4, 1862 in Saline County by Captain Alexander

A. G/B. Barnes, Private, 9th Texas Artillery, joined for duty and enrolled May 13, 1862 at Tulip, Arkansas by Geo. D. Alexander for the war