Invitation only is a crap shoot because it varies between different teams in different years. There are some teams that use the main camp as an actual test of the roster. Most of the players they expect to make the roster are there, and those players are almost exclusively ones the team has drafted, tendered, protected, or scouted extensively. But there are others who only host an invitation-only main camp without a predraft camp, I can think of Shreveport recently as one, where it is likely a big money maker with a couple hundred skaters on a couple of teams. There is no streamlined and standard answer, teams just have different development systems and it changes constantly over time. A NAHL team owner who also owns NA3HL teams, which is quite normal, will likely hand out invitations to players on those NA3HL teams in order to help sell the organizational and league development model to players. But other teams might not need it and don't want the hassle of organizing 3 camps in the summer when they already know the roster for next season and want to enjoy their summer.