You should identify any security protocols for the venue by speaking with building management. This is especially important for Saturday and Sunday, when many venues would normally be locked. Things to consider:
Make sure the venue will be accessible after hours and on weekends.
Determine any special check-in procedures for participants, which may include providing a list of participants to building security prior to the event.
Signage in the lobby of the building to direct participants to the correct floor or area of the building.