"Additional RR features including:
An Info_Spectate brush entity to join the spectators.
An Info_Gameplay point entity to tell NS what species team one and two are.
An Info_Team_AutoAssign brush entity to act as a random team join.
32 player starts at least 128 units apart to prevent telefragging.
Team entrances that are wide enough for 2 or more players to pass side by side. "
-from ChromeAngel's NS mapping guide
if you still need to know how to create each brush just ask.
and on the actual signs for team joins:
"The marine and alien team joins are each made of 2 brush entites, an info_team_join (actually makes the player join the team) and a func_illusionary (for the sign itself). The func_illusionary is textured with null on all but 1 side the remaining side is textured with the appropriate sign from the NS.WAD. The func_illsionary has it's RenderFX set to distort, RenderMode additive, RenderAmount 255. it doesn't matter how you texture the info_team_join it is invisble in-game."
hope that helped more than my earlier comment