There are several different types of scrims. The scrim seasons are held, on average, three times a year. They are a tournament/league style regular season with a bracketed post season play off tournament to determine the season champ. These are what we usually refer to as Captain's scrims. They are open to members of BFM only whether they be big tags or small tags.
There are also 'for fun' scrims held nightly. These are like pick up games on the playground. They can happen almost anytime as long as there are enough people to play and an officer available to organize it. The organizing officer picks two people to be team captains and they pick their teams from the players available and get to it. These scrims are open to anyone who has access to BFM vent including guests without tags.
There are also more specialized scrims that are done. Some folks like to do 2V2 scrims, which are usually only done as a 'for fun' scrims but some of the Aussies are trying to get a scrim season together for that game type now.
We also have a scrim season for our Halo Soccer game type.
Then there are, on rare occasions, scrims that will include King of the Hill, Odd Ball, Team Slayer and CTF game types that are almost always 'for fun' scrims. The one thing in common to all of our scrims though are that you must be on BFM vent to participate.