I believe that the group in question is the New Black Panthers. They are asking for a group of 5000 volunteers to find and capture Zimmerman, and a reward of $10k to the person who does.
Biggest racist group in history. Makes the Klan look like saints.
Najee Muhammad, a member of the
New Black Liberation Militia, announced Thursday that members of the group plan to take the 28-year-old Zimmerman to federal authorities since local police haven't acted. - from link in OP - There is no mention of citizen's arrests in Florida's criminal statutes but state courts have cited common law in establishing when such an arrest can be made. The crime has to be a felony.
It has to be witnessed by the citizen making the arrest or the citizen has to have probable cause that the person being arrested is guilty of a felony.
are some militias created more than equal?
I'm quietly confident that citing a newspaper article misstating facts does not amount to 'probable cause'. Anyone who attempts a 'citizens arrest' is likely to end up charged with unlawful detainment, and possibly kidnapping charges. That is assuming they hand him over alive.... if not, well, that would be first degree murder, I suspect.
Overall, not a very smart move by the NBP.