MonksRule, I would strongly suggest that you take Yoshimo with you instead of Jan, and then you can take a six-person party. I won't spoil you, but believe me, the roleplaying and storyline will be improved by it. After you return from Spellhold, you can pick up Jan again...
(You could also swap Yoshi for another member of your party and keep both Jan and Yoshimo. In short, if you take Yoshimo with you, you can take six people and still manage to get Imoen.... no spoiling, just go and find out for yourself!

)
BTW sorry for not even welcoming you to our wonderful forum!!! ~slaps herself~
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Melusine, Archbabe of the Order of the Holy Flame and the Laughing Hyenas, &
Official Entertainer Elf of the BG2 Bar
Your voice is ambrosia