Intersystem grounding bus ran out of terminals; now what?
Nothing in 250.94 is intended to prohibit multiple IBT bars, so get another one and toss it on the same conductor the first one is attached to

The concept behind the presence of 250.94 in the NEC and its requirement for an intersystem bonding termination (IBT) is to provide installers of auxiliary systems (such as telecom, antennas, alarms, and such) a convenient, centrally-located place to land the bonding conductor that connects their ground point (ground block, protector, or auxiliary ground rod) to the electrical system ground.

As a result, and according to this ASHI Reporter article, nothing in 250.94 prohibits multiple IBTs from being present in a building.  In fact, there are situations where fitting a second IBT would be encouraged to avoid telecom installers giving up on reaching an IBT simply because they are not equipped to reach the existing one from where they are at.  As the article states:


  The author suggests that a best-practice IBT deployment would include an interior IBT device at the service equipment location — typically the main electrical panel — and an exterior IBT device at the service entrance location. Such a plan will accommodate almost every possible grounding and bonding scenario for one- and two-family residential dwellings. The Code requires only one such device, preferably mounted on the exterior in the case of residential structures.


So, you can simply get another GBB5 (or equivalent IBT device) and attach it using the lay-in lug to the same grounding electrode conductor the existing one is attached to, then attach the unterminated communications bonding wires to the new IBT.