I just found out yesterday that getting advantage from just being adjacent is optional rule (with only one barbarian totem getting it normally) - what I remembered was a combination of 3.5 and 5e things:
3.5 rogue needs flanking for SA and 5e explicitly stating that rogue needs only an ally adjacent to a creature to get SA...somehow, I mixed up those two.
So, for the record, you can grant advantage IF YOU SPEND ACTION to aid a companion. Even being in true flanking position gives no bonuses (unless optional rules are used or DM rules otherwise)
Sorry for any confusion I caused.