Roger Joseph Manning Jr. plays on all recordings by Jellyfish. Eric Dover appears on a couple of them.
For complete details (including some other Roger appearances not mentioned here), consult Chris Bray's Jellyfish Discography.
In the film "Born to be Wild", there is a song creditied to the band
"Doverman", and written by Eric Dover and Roger Manning. It's unclear whether they actually appear on the soundtrack album (BMG/MILAN 3138 35705 4).
In "The Brady Bunch Movie" (1995), Roger Joseph Manning Jr. and Eric Dover appear as part of the band backing singer Davy Jones near the end of the film. It's unclear whether they actually appear on the soundtrack album (BMG/MILAN 3138 35698 4).