Quoting GoaFan77, reply 1Are you sure you don't have locked teams enabled?
God damn it! Yes, yes i think I did.
*Facepalm*
...so if you can't use envoys before getting cease fire, what's the best way to bolster relations until that point?
Well you technical can, as you were doing, it's just unless the planet is defense less you're going to have that same problem of them shooting at you.
Your general idea was pretty good. Getting those diplomatic bonus techs fairly early is a must. Then you'll want to pick a neighbor you want to get rid of and starting destroying his stuff. Military relations are the main thing that makes or brakes alliances. Attacking someone else's enemy will get you on their good side. This also means if you want to make an alliance with an AI that's attacking you, you're probably out of luck. Also note that depending on the factions involved, getting an alliance early maybe impossible. The TEC Rebels for example really hate anyone who isn't TEC, and the Vasari Loyalists aren't particularly friendly to anyone but themselves either.
Your first goal is to get relations to 2.5, so you can get a trade agreement. Assuming you have trade ports, your relations should go up slowly but steadily thanks to the trade modifier. If you have the needed relationship with them for a ceasefire but they don't really like you, sending them resources is a very good way to get a ceasefire quickly. After you have it you can quickly cement the alliance with envoys.