Turn on Available to merc on your account page, or use /merc available in Discord, and you are on the Mercs list for 14 days. You need Discord and Steam linked. You can be on a team already: the list is for playing for other teams.
A Team Leader or Team Rep can ask you from the list. You get a line in the bell and a Discord message with Accept and Decline, and 48 hours to answer. Silence counts as no.
A yes lasts 7 days. You stay on your own team, you are not added to theirs, and the rule of one team per person does not count it. The team gets your Discord name so they can reach you. Their Team Leader or you can end it early.
Nobody can be asked who has not opted in. A team can send 3 requests in 24 hours, and after a no it waits 24 hours before asking the same person again.