For joining friends it’s instant you usually spawn in the system as the host. For jumps you could taxi your friends with you in a capital ship, or you could warp to them by exiting the space anomaly. Exploring an uncharted system? warp to them via anomaly. They wanna show you their base across the galaxy? Warp to them via anomaly. They’re in a different galaxy? Believe it or not, anomaly

Yeah in multiplayer you still have access to all the warp points such as your bases, previously visited stations. SOMETIMES my friends (it’s been a long time since I’ve done multiplayer) could see my bases otherwise we just anomaly warp